Added util/python/BUILD to provide //util/python:python_headers dependency for…

Added util/python/BUILD to provide //util/python:python_headers dependency for building use_fast_cpp_protos

util/python/BUILD

+# This is a placeholder for python headers. Projects needing to use
+# fast cpp protos in protobuf's python interface should build with
+# --define=use_fast_cpp_protos=true, and in addition, provide
+# //util/python:python_headers dependency that in turn provides Python.h.
+cc_library(
+    name = "python_headers",
+    visibility = ["//visibility:public"],
+)