key is already used with const, doubling it confuses libc++

Index: src/tools/radiant/radiant/ui/colourscheme/ColourSchemeManager.h
--- src/tools/radiant/radiant/ui/colourscheme/ColourSchemeManager.h.orig
+++ src/tools/radiant/radiant/ui/colourscheme/ColourSchemeManager.h
@@ -7,7 +7,7 @@
 namespace ui {
 
 // A map storing all the ColourSchemes with the name as key
-typedef std::map<const std::string, ColourScheme> ColourSchemeMap;
+typedef std::map<std::string, ColourScheme> ColourSchemeMap;
 
 class ColourSchemeManager {
 	private:
