Constify XCode generator getters to match cmGlobalGenerator

Commit 8a0eb78f (Constify many getters of cmGlobalGenerator, 2011-03-26)
added const qualifiers to many cmGlobalGenerator methods.  Fix the
signature of the virtual function overrides in cmGlobalXCodeGenerator to
match.

Source/cmGlobalXCodeGenerator.h

@@ -194,8 +194,8 @@ private:
   void AppendFlag(std::string& flags, std::string const& flag);
 
 protected:
-  virtual const char* GetInstallTargetName()      { return "install"; }
-  virtual const char* GetPackageTargetName()      { return "package"; }
+  virtual const char* GetInstallTargetName() const { return "install"; }
+  virtual const char* GetPackageTargetName() const { return "package"; }
 
   unsigned int XcodeVersion;
   std::string VersionString;