reduce some include dependencies

rtengine/color.h

@@ -20,7 +20,6 @@
 #pragma once
 
 #include <array>
-#include <glibmm/ustring.h>
 
 #include "rt_math.h"
 #include "LUT.h"
@@ -30,6 +29,13 @@
 
 #define SAT(a,b,c) ((float)max(a,b,c)-(float)min(a,b,c))/(float)max(a,b,c)
 
+namespace Glib
+{
+
+class ustring;
+
+}
+
 namespace rtengine
 {