cmLocalGenerator: Constify target in API.

Source/cmLocalGenerator.cxx

@@ -1180,7 +1180,7 @@ void cmLocalGenerator::AddCompileOptions(
 
 //----------------------------------------------------------------------------
 void cmLocalGenerator::GetIncludeDirectories(std::vector<std::string>& dirs,
-                                             cmGeneratorTarget* target,
+                                             cmGeneratorTarget const* target,
                                              const std::string& lang,
                                              const std::string& config,
                                              bool stripImplicitInclDirs

Source/cmLocalGenerator.h

@@ -171,7 +171,7 @@ public:
 
   /** Get the include flags for the current makefile and language.  */
   void GetIncludeDirectories(std::vector<std::string>& dirs,
-                             cmGeneratorTarget* target,
+                             cmGeneratorTarget const* target,
                              const std::string& lang = "C",
                              const std::string& config = "",
                              bool stripImplicitInclDirs = true) const;