diffutils: fix configure cache issue with BR2_NEEDS_GETTEXT

Like flex, diffutils doesn't NEED gettext/libintl, but it's configure script
checks for it, so make sure those a built before diffutils, otherwise it
will populate tgt-config.cache with invalid values, breaking the build of
other packages needing it (like libglib2).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>