cmLocalGenerator: Constify GetIncludeDirectories method.

Source/cmLocalGenerator.cxx

@@ -1341,7 +1341,7 @@ void cmLocalGenerator::GetIncludeDirectories(std::vector<std::string>& dirs,
                                              const std::string& lang,
                                              const std::string& config,
                                              bool stripImplicitInclDirs
-                                            )
+                                            ) const
 {
   // Need to decide whether to automatically include the source and
   // binary directories at the beginning of the include path.

Source/cmLocalGenerator.h

@@ -183,7 +183,7 @@ public:
                              cmGeneratorTarget* target,
                              const std::string& lang = "C",
                              const std::string& config = "",
-                             bool stripImplicitInclDirs = true);
+                             bool stripImplicitInclDirs = true) const;
   void AddCompileOptions(std::string& flags, cmTarget* target,
                          const std::string& lang, const std::string& config);
   void AddCompileDefinitions(std::set<std::string>& defines,