afs: Use literal NULL for NULL function pointer

The clang compiler complains that this function pointer cast is not a
prototype.

   .../osi_file.c:141:27: error: this function declaration is not a
      prototype [-Werror,-Wstrict-prototypes]
    afile->proc = (int (*)())0;
                          ^
                           void

Just use NULL instead of trying to create a cast to 0.

Change-Id: I3adb22cb38db8c854d9313ec9eb2b008a4e9a9db
Reviewed-on: https://gerrit.openafs.org/14902
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/afs/LINUX/osi_file.c

@@ -138,7 +138,7 @@ osi_UFSOpen(afs_dcache_id_t *ainode)
     }
 
     afile->offset = 0;
-    afile->proc = (int (*)())0;
+    afile->proc = NULL;
     return (void *)afile;
 }