Make //:protobuf_python have correct __init__.py.

Previously //:protobuf_python set no __init__.py so Bazel created an
empty one. This change makes it use the __init__.py from the repository.
diff --git a/BUILD b/BUILD
index bf89462..d63c6d4 100644
--- a/BUILD
+++ b/BUILD
@@ -693,6 +693,7 @@
         ":python_srcs",
         "//external:six",
     ],
+    py_extra_srcs = glob(["python/**/__init__.py"]),
     srcs_version = "PY2AND3",
     visibility = ["//visibility:public"],
 )