Add golang-protobuf

soong_ui now requires it.

Change-Id: Ie2227a19b48141e6f5b8249eb3c61016e2c6ac33

default.xml

@@ -18,6 +18,7 @@
     <linkfile src="root.bp" dest="Android.bp" />
     <linkfile src="bootstrap.bash" dest="bootstrap.bash" />
   </project>
+  <project path="external/golang-protobuf" name="platform/external/golang-protobuf" />
   <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" clone-depth="1" />
   <project path="prebuilts/clang-tools" name="platform/prebuilts/clang-tools" clone-depth="3" />