ceph: trivial buildbot warnings fix

The linux-next build bot found a three of warnings, this addresses all of them.

 * non-ANSI function declaration of function 'ceph_fscache_register' and
   'ceph_fscache_unregister'
 * symbol 'ceph_cache_netfs' was not declared, now it's extern in the header.
 * warning: "pr_fmt" redefined

Signed-off-by: Milosz Tanski <milosz@adfin.com>

fs/ceph/cache.h

@@ -26,6 +26,8 @@
 
 #ifdef CONFIG_CEPH_FSCACHE
 
+extern struct fscache_netfs ceph_cache_netfs;
+
 int ceph_fscache_register(void);
 void ceph_fscache_unregister(void);