Adding deps to protobuf rules.

Change-Id: Ib1f4f1e061a55885389a7b51c98317ee241a1bba
diff --git a/build_tests/proto.proto b/build_tests/proto.proto
index 367a650..d0e6fe2 100644
--- a/build_tests/proto.proto
+++ b/build_tests/proto.proto
@@ -4,9 +4,13 @@
 
 import "google/protobuf/empty.proto";
 
+import "build_tests/proto_base.proto";
+
 message TestProto {
   string s = 1;
   int32 i = 2;
   // Making sure that well-known protos work.
   .google.protobuf.Empty empty = 3;
+  // Making sure we can depend on other protos.
+  BaseProto base_proto = 4;
 }