Add std:: to strings in Generator.cc/hh (#129)

Signed-off-by: Hill Ma <hillma@google.com>

Author: mahiuchun

src/Generator.cc

@@ -67,7 +67,7 @@ Generator::~Generator()
 
 /////////////////////////////////////////////////
 bool Generator::Generate(const FileDescriptor *_file,
-                               const string &/*_parameter*/,
+                               const std::string &/*_parameter*/,
                                OutputDirectory *_generatorContext,
                                std::string * /*_error*/) const
 {

src/Generator.hh

@@ -44,9 +44,9 @@ class Generator : public CodeGenerator
   /// \param[in] _generatorContext Output directory.
   /// \param[in] _error Unused string value
   public: virtual bool Generate(const FileDescriptor *_file,
-              const string &_parameter,
+              const std::string &_parameter,
               OutputDirectory *_generatorContext,
-              string *_error) const;
+              std::string *_error) const;
 
   // private: GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(Generator);
 };