Update SDWebImagePrefetecher.m

I added "+ self.skippedCount" because it never completes if there are skipped operations.

@@ -82,7 +82,7 @@
         if (self.prefetchURLs.count > self.requestedCount) {
             [self startPrefetchingAtIndex:self.requestedCount];
         }
-        else if (self.finishedCount == self.requestedCount) {
+        else if (self.finishedCount + self.skippedCount == self.requestedCount) {
             [self reportStatus];
             if (self.completionBlock) {
                 self.completionBlock(self.finishedCount, self.skippedCount);