cmGeneratorTarget: Make GetIncludeDirectories const.

Source/cmGeneratorTarget.cxx

@@ -611,8 +611,8 @@ const char* cmGeneratorTarget::GetCreateRuleVariable() const
 }
 
 //----------------------------------------------------------------------------
-std::vector<std::string> cmGeneratorTarget::GetIncludeDirectories(
-                                                          const char *config)
+std::vector<std::string>
+cmGeneratorTarget::GetIncludeDirectories(const char *config) const
 {
   return this->Target->GetIncludeDirectories(config);
 }

Source/cmGeneratorTarget.h

@@ -70,7 +70,7 @@ public:
   const char* GetCreateRuleVariable() const;
 
   /** Get the include directories for this target.  */
-  std::vector<std::string> GetIncludeDirectories(const char *config);
+  std::vector<std::string> GetIncludeDirectories(const char *config) const;
 
   bool IsSystemIncludeDirectory(const char *dir, const char *config) const;