misc: Use the protobuf version to choose between ByteSize(Long).

Older versions of protobuf are apparently still what's provided with
some current versions of some distributions. To avoid breaking the build
on those distributions and to avoid an annoying deprecation warning on
newer systems, check GOOGLE_PROTOBUF_VERSION to choose which to use.

According to the CHANGES.txt file here:

https://github.com/protocolbuffers/protobuf/blob/master/CHANGES.txt

The new ByteSizeLong function was introduced in version 3.1.0 in 2016.

Change-Id: I7f8eeda144bf9556d17d7a0a741996f3137b48b4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28287
Reviewed-by: Tommaso Marinelli  <tommarin@ucm.es>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/proto/protoio.cc

@@ -87,7 +87,12 @@ ProtoOutputStream::write(const Message& msg)
     io::CodedOutputStream codedStream(zeroCopyStream);
 
     // Write the size of the message to the stream
-    codedStream.WriteVarint32(msg.ByteSizeLong());
+#   if GOOGLE_PROTOBUF_VERSION < 3001000
+        auto msg_size = msg.ByteSize();
+#   else
+        auto msg_size = msg.ByteSizeLong();
+#   endif
+    codedStream.WriteVarint32(msg_size);
 
     // Write the message itself to the stream
     msg.SerializeWithCachedSizes(&codedStream);