glob: silence -Wattribute warnings

Colin Watson reported that some versions of gcc warn about
attribute leaf used on a static function, since it has no
effect on anything but external functions.

* posix/glob.c (next_brace_sub, prefix_array, collated_compare):
Use __THROWNL rather than __THROW on static functions.

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