fix a const warning.

widget/src/mac/nsDragService.cpp

@@ -699,7 +699,7 @@ nsDragService :: GetDataForFlavor ( nsISupportsArray* inDragItems, DragReference
     
     // if someone was asking for text/plain, lookup unicode instead so we can convert it.
     PRBool needToDoConversionToPlainText = PR_FALSE;
-    char* actualFlavor = mimeFlavor;
+    const char* actualFlavor = mimeFlavor;
     if ( strcmp(mimeFlavor,kTextMime) == 0 ) {
       actualFlavor = kUnicodeMime;
       needToDoConversionToPlainText = PR_TRUE;