Now wrapping match-fstype to avoid compilation when not needed

[Chris Rees]

Hi again!

I've got a patch to libbb/match_fstype.c this time; it gets compiled
even if it isn't needed.

http://www.bayofrum.net/~chris/patches/busybox-match-fstype.patch

This is a problem on FreeBSD for example which doesn't have mntent.h
or the stuff defined.

I don't expect you to take the patch as-is, but I'm not familiar
enough with the way menuconfig works to make it 'clean', as it were.

Chris