proto: Add a missing std:: in protoio.cc.

Also include string which that file uses.

Change-Id: I79cd9c88652abdd1d16b3368dd37268509f4d3f0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60310
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

src/proto/protoio.cc

@@ -37,6 +37,8 @@
 
 #include "proto/protoio.hh"
 
+#include <string>
+
 #include "base/logging.hh"
 
 using namespace google::protobuf;
@@ -53,7 +55,7 @@ ProtoOutputStream::ProtoOutputStream(const std::string& filename) :
     // wrapped in a gzip stream if the filename ends with .gz. The
     // latter stream is in turn wrapped in a coded stream
     wrappedFileStream = new io::OstreamOutputStream(&fileStream);
-    if (filename.find_last_of('.') != string::npos &&
+    if (filename.find_last_of('.') != std::string::npos &&
         filename.substr(filename.find_last_of('.') + 1) == "gz") {
         gzipStream = new io::GzipOutputStream(wrappedFileStream);
         zeroCopyStream = gzipStream;