drw: drw_scm_create: use Clr type

in this context XftColor is a too low-level type.

drw.c

@@ -200,7 +200,7 @@ drw_scm_create(Drw *drw, const char *clrnames[], size_t clrcount)
 	Clr *ret;
 
 	/* need at least two colors for a scheme */
-	if (!drw || !clrnames || clrcount < 2 || !(ret = ecalloc(clrcount, sizeof(XftColor))))
+	if (!drw || !clrnames || clrcount < 2 || !(ret = ecalloc(clrcount, sizeof(Clr))))
 		return NULL;
 
 	for (i = 0; i < clrcount; i++)