libunarchive: fix build failure with !FEATURE_TAR_UNAME_GNAME

libunarchive: fix build failure with !FEATURE_TAR_UNAME_GNAME

We can't use C if(...) with ENABLE_FEATURE_TAR_UNAME_GNAME because it
relies on conditional members in the file_header_t structure:

archival/libunarchive/data_extract_all.c: In function ‘data_extract_all’:
archival/libunarchive/data_extract_all.c:123: error: ‘file_header_t’ has no member named ‘uname’
archival/libunarchive/data_extract_all.c:124: error: ‘file_header_t’ has no member named ‘uname’
archival/libunarchive/data_extract_all.c:127: error: ‘file_header_t’ has no member named ‘gname’
archival/libunarchive/data_extract_all.c:128: error: ‘file_header_t’ has no member named ‘gname’
make[1]: *** [archival/libunarchive/data_extract_all.o] Error 1

Signed-off-by: Mike Frysinger <vapier@gentoo.org>