Fix warning about conversion on mingw64

src/nvim/api/buffer.c

@@ -399,7 +399,11 @@ void nvim_buf_set_lines(uint64_t channel_id,
   // Only adjust marks if we managed to switch to a window that holds
   // the buffer, otherwise line numbers will be invalid.
   if (save_curbuf.br_buf == NULL) {
-    mark_adjust((linenr_T)start, (linenr_T)(end - 1), MAXLNUM, extra, false);
+    mark_adjust((linenr_T)start,
+                (linenr_T)(end - 1),
+                MAXLNUM,
+                (long)extra,
+                false);
   }
 
   changed_lines((linenr_T)start, 0, (linenr_T)end, (long)extra);