platform_macros.h: #undef GOOGLE_PROTOBUF_PLATFORM_ERROR once it's no longer needed

src/google/protobuf/stubs/platform_macros.h

@@ -89,4 +89,6 @@ GOOGLE_PROTOBUF_PLATFORM_ERROR
 #define GOOGLE_PROTOBUF_OS_NACL
 #endif
 
+#undef GOOGLE_PROTOBUF_PLATFORM_ERROR
+
 #endif  // GOOGLE_PROTOBUF_PLATFORM_MACROS_H_