glob: fix build for platforms without __THROW

glob: fix build for platforms without __THROW

The previous fix for glibc broke builds on non-glibc platforms
that don't pre-define __THROW.  Perhaps we could make the code
more optimal by using gcc attributes instead of expanding to
nothing, to match the glibc usage; but for now this fixes the
build.  Reproted by Tom G. Christensen.

* lib/glob.in.h (__THROW): Add definition again.

Signed-off-by: Eric Blake <eblake@redhat.com>