Fixed printf/gpr_log format.

The faulty line is behind a preprocessor define that isn't on by default.

src/core/ext/client_config/lb_policy.c

@@ -60,8 +60,9 @@ static gpr_atm ref_mutate(grpc_lb_policy *c, gpr_atm delta,
                             : gpr_atm_no_barrier_fetch_add(&c->ref_pair, delta);
 #ifdef GRPC_LB_POLICY_REFCOUNT_DEBUG
   gpr_log(file, line, GPR_LOG_SEVERITY_DEBUG,
-          "LB_POLICY: %p % 12s 0x%08x -> 0x%08x [%s]", c, purpose, old_val,
-          old_val + delta, reason);
+          "LB_POLICY: 0x%" PRIxPTR " %12s 0x%" PRIxPTR " -> 0x%" PRIxPTR
+          " [%s]",
+          (intptr_t)c, purpose, old_val, old_val + delta, reason);
 #endif
   return old_val;
 }