Close the correct file when using a metalink.xml file

When using librepo, PackageKit crashes after after issuing a few hundred
transactions with 'Too many open files' -- Sure enough, looking at the output
of lsof before it crashes, I have hundreds of open files to various metalink.xml
files.

Fix the typo so that the metalink file descriptor actually gets closed.

librepo/handle.c

@@ -110,7 +110,7 @@ lr_handle_free(LrHandle *handle)
     if (handle->mirrorlist_fd != -1)
         close(handle->mirrorlist_fd);
     if (handle->metalink_fd != -1)
-        close(handle->mirrorlist_fd);
+        close(handle->metalink_fd);
     lr_handle_free_list(&handle->urls);
     lr_free(handle->fastestmirrorcache);
     lr_free(handle->mirrorlist);