Fix dragging GUI tabs resulting in a crash

This was introduced by macvim-dev#1457 which was supposed to be a simple compiler
warning cleanup but it introduced a signed integer bug. Fix that here.

Fix macvim-dev#1473

src/MacVim/PSMTabBarControl/source/PSMTabDragAssistant.m

@@ -386,7 +386,7 @@ - (void)distributePlaceholdersInTabBar:(PSMTabBarControl *)control
 
 - (void)removeAllPlaceholdersFromTabBar:(PSMTabBarControl *)control
 {
-    NSUInteger i, cellCount = [[control cells] count];
+    int i, cellCount = (int)[[control cells] count];
     for(i = (cellCount - 1); i >= 0; i--){
         PSMTabBarCell *cell = [[control cells] objectAtIndex:i];
         if([cell isPlaceholder])