include/libbb.h: mark malloc-returning functions with GCC attribute

include/libbb.h: mark malloc-returning functions with GCC attribute

function                                             old     new   delta
ash_main                                            1336    1365     +29
readcmd                                             1062    1072     +10
add_split_dependencies                               707     717     +10
rtnl_talk                                            539     548      +9
normalize                                            165     174      +9
xmalloc_optname_optval                               583     590      +7
test_main                                            247     253      +6
insert_bg_job                                        366     371      +5
dname_enc                                            391     395      +4
nslookup_main                                        165     168      +3
ifplugd_main                                        1099    1102      +3
tftpd_main                                           493     495      +2
make_new_session                                     413     415      +2
localcmd                                             274     276      +2
date_main                                            696     698      +2
tee_main                                             317     318      +1
setvar                                               183     184      +1
hash_find                                            233     234      +1
xrtnl_dump_filter                                    400     399      -1
prepend_new_eth_table                                297     295      -2
unpack_Z_stream                                     1229    1226      -3
read_line_input                                     4823    4820      -3
do_load                                              523     520      -3
udhcp_run_script                                     669     665      -4
mkfs_vfat_main                                      1609    1605      -4
rtnl_rtprot_initialize                                70      65      -5
procps_scan                                         1842    1835      -7
read_opt                                             873     865      -8
ifupdown_main                                       2133    2125      -8
mkfs_ext2_main                                      2492    2482     -10
logread_main                                         503     492     -11
svlogd_main                                         1441    1429     -12
rewrite                                             1032    1018     -14
fbsplash_main                                        917     903     -14
str2sockaddr                                         431     405     -26
unpack_lzma_stream                                  2705    2677     -28
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 18/18 up/down: 106/-163)        Total: -57 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>