Fix bad signed/unsigned comparisons

Either by casting, or switching to a more appropriate type
for the variable.
diff --git a/win/rfb_win32/CleanDesktop.cxx b/win/rfb_win32/CleanDesktop.cxx
index 52dc6bd..bad95f0 100644
--- a/win/rfb_win32/CleanDesktop.cxx
+++ b/win/rfb_win32/CleanDesktop.cxx
@@ -129,7 +129,7 @@
         vlog.error("failed to get desktop item count: %ld", hr);
         return false;
       }
-      for (unsigned int i=0; i<itemCount; i++) {
+      for (int i=0; i<itemCount; i++) {
         if (enableItem(i, false))
           restoreItems.insert(i);
       }