Fixing includes inconsistency.

src/compiler/cpp_generator.cc

@@ -31,6 +31,9 @@
  *
  */
 
+#include <string>
+#include <map>
+
 #include "src/compiler/cpp_generator.h"
 
 #include "src/compiler/cpp_generator_helpers.h"

src/compiler/cpp_plugin.cc

@@ -35,6 +35,8 @@
 //
 
 #include <memory>
+#include <string>
+
 #include "src/compiler/cpp_generator.h"
 #include "src/compiler/cpp_generator_helpers.h"
 #include <google/protobuf/descriptor.h>

src/compiler/ruby_generator.cc

@@ -32,6 +32,7 @@
  */
 
 #include <cctype>
+#include <string>
 #include <map>
 #include <vector>
 

src/compiler/ruby_plugin.cc

@@ -37,6 +37,7 @@
 // and net/proto2/compiler/public/plugin.h for more information on plugins.
 
 #include <memory>
+#include <string>
 
 #include "src/compiler/ruby_generator.h"
 #include "src/compiler/ruby_generator_helpers-inl.h"