build.gradle: Add missing dependsOn for generated code sync
build.gradle: Add missing dependsOn for generated code sync

When messing with error prone for another commit, Gradle started
producing a clear warning the dependsOn was missing. But the warning
was not reliable. However, even when no warning was printed it is clear
the task was broken.
stanley-cheung/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java