VS: Emit "utf-8" encoding as lowercase in .vcxproj headers

Match the XML preamble generated by VS 2010 and later.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -951,6 +951,11 @@ void cmGlobalVisualStudio10Generator::PathTooLong(cmGeneratorTarget* target,
   }
 }
 
+std::string cmGlobalVisualStudio10Generator::Encoding()
+{
+  return "utf-8";
+}
+
 bool cmGlobalVisualStudio10Generator::IsNsightTegra() const
 {
   return !this->NsightTegraVersion.empty();

Source/cmGlobalVisualStudio10Generator.h

@@ -96,6 +96,7 @@ public:
   void PathTooLong(cmGeneratorTarget* target, cmSourceFile const* sf,
                    std::string const& sfRel);
 
+  std::string Encoding() override;
   virtual const char* GetToolsVersion() { return "4.0"; }
 
   bool FindMakeProgram(cmMakefile* mf) override;