mkimage: Fix missing free() in show_valid_options()

The allocated memory should be freed. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 150963)
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/mkimage.c b/tools/mkimage.c
index 02f88bb..0c6dba8 100644
--- a/tools/mkimage.c
+++ b/tools/mkimage.c
@@ -64,6 +64,7 @@
 			genimg_get_cat_name(category, item));
 	}
 	fprintf(stderr, "\n");
+	free(order);
 
 	return 0;
 }