Add protobuf_clib to deps of grpc++_codegen_lib

This is required in order to compile grpc++_codegen_lib.
diff --git a/BUILD b/BUILD
index 025878d..fc4dfe8 100644
--- a/BUILD
+++ b/BUILD
@@ -1041,6 +1041,7 @@
     ".",
   ],
   deps = [
+    "//external:protobuf_clib",
   ],
 )