Use wxColour ctor from NSColor directly

This makes the code simpler and fixes compilation with 10.7 SDK which doesn't
have CGColor property in NSColor.

src/osx/cocoa/settings.mm

@@ -116,10 +116,7 @@ wxColour wxSystemSettingsNative::GetColour(wxSystemColour index)
 
     if ( sysColor )
     {
-        CGColorRef cgCol = sysColor.CGColor;
-        // wxColour takes ownership of CF reference
-        CFRetain(cgCol);
-        return wxColour(cgCol);
+        return wxColour(sysColor);
     }
     else
     {