util/glsl2spirv: fix appending extra flags

The variable is called `extra`, but what's written is `extra - flags`,
and `flags` is undefined, so if the variable was ever passed there would
be an uncaught exception.

fixes: 9786d9ef2a

Reviewed-by: Luis Felipe Strano Moraes <luis.strano@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19449>

src/util/glsl2spirv.py

@@ -136,7 +136,7 @@ def process_file(args):
             cmd_list += ["--variable-name", args.vn]
 
         if args.extra is not None:
-            cmd_list.append(args.extra-flags)
+            cmd_list.append(args.extra)
 
         if args.create_entry is not None:
             cmd_list += ["--entry-point", args.create_entry]