libceph: make pgid_cmp() global

calc_target() code is going to need to know how to compare PGs.  Take
lhs and rhs pgid by const * while at it.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/include/linux/ceph/osdmap.h b/include/linux/ceph/osdmap.h
index 3fd978a..7783237 100644
--- a/include/linux/ceph/osdmap.h
+++ b/include/linux/ceph/osdmap.h
@@ -24,6 +24,8 @@
 	uint32_t seed;
 };
 
+int ceph_pg_compare(const struct ceph_pg *lhs, const struct ceph_pg *rhs);
+
 #define CEPH_POOL_FLAG_HASHPSPOOL  1
 
 struct ceph_pg_pool_info {