fnmatch: avoid compiler warning

fnmatch: avoid compiler warning

Detected on Ubuntu 10.04, where the glibc fnmatch fix is not yet
present; also reproduced via:
$ gl_cv_func_fnmatch_posix=no CFLAGS=-Wall \
    ./gnulib-tool --with-tests --test fnmatch

In file included from gllib/fnmatch.c:172:0:
gllib/fnmatch_loop.c: In function ‘internal_fnmatch’:
gllib/fnmatch_loop.c:203:28: error: initialization discards qualifiers from pointer target type
gllib/fnmatch_loop.c:204:28: error: initialization discards qualifiers from pointer target type

* lib/fnmatch_loop.c (FCT): Use correct type.
Reported by Matthias Bolte.

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