ci: fix gcc-windows -std=c99 (#25534)

Author: kbkpbot

.github/workflows/windows_ci.yml

@@ -59,7 +59,7 @@ jobs:
       - name: Test new v.c
         run: |
           v -o v.c cmd/v
-          gcc -Werror -municode -w v.c -lws2_32
+          gcc -Werror -municode -w v.c -lws2_32 -std=c99
       - name: Install dependencies
         run: |
           v retry -- v setup-freetype