rpm2cpio: use rpm_gettags

rpm2cpio: use rpm_gettags

function                                             old     new   delta
rpm_gettags                                            -     451    +451
rpm2cpio_main                                        140      78     -62
skip_header                                           92       -     -92
rpm_main                                            1477    1049    -428
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/2 up/down: 451/-582)         Total: -131 bytes

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