tar: add IF_FEATURE_* checks

tar: add IF_FEATURE_* checks

A following linking error was observed:
| ==========
| archival/lib.a(tar.o): In function `tar_main':
| archival/tar.c:1168: undefined reference to `unpack_Z_stream'
| archival/tar.c:1168: undefined reference to `unpack_Z_stream'
| ld: busybox_unstripped: hidden symbol `unpack_Z_stream' isn't defined
| ld: final link failed: Bad value

this happened with clang compiler, with the following configs:
| CONFIG_TAR=y
| # CONFIG_FEATURE_SEAMLESS_Z is not set

which can be fixed by adding IF_FEATURE_* checks in.

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>