busybox

bzcat: compile bunzip2_main() if BZCAT.

Currently, BZCAT (and BUNZIP2) selects FEATURE_BZIP2_DECOMPRESS,
thus, "#if ENABLE_FEATURE_BZIP2_DECOMPRESS" around bunzip2_main()
is sufficient. But let's robustify it.

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