Fix gRPC compilation failure

Update location of rpc_method.cc on the Makefile
	Change on 2015/01/12 by rsilvera <rsilvera@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=83786942
diff --git a/build.json b/build.json
index 968c1df..219ff23 100644
--- a/build.json
+++ b/build.json
@@ -335,7 +335,7 @@
         "src/cpp/client/credentials.cc",
         "src/cpp/client/internal_stub.cc",
         "src/cpp/proto/proto_utils.cc",
-        "src/cpp/rpc_method.cc",
+        "src/cpp/common/rpc_method.cc",
         "src/cpp/server/async_server.cc",
         "src/cpp/server/async_server_context.cc",
         "src/cpp/server/completion_queue.cc",