tmux: use -std=gnu17 to fix ftbfs with gcc 15 (#57612)

use `-std=gnu17` for now

![image](https://github.com/user-attachments/assets/0acf0d7c-2ad6-47d7-8340-0aa7f0f04310)

Signed-off-by: Tuan Anh Tran <me@tuananh.org>

Author: tuananh

tmux.yaml

@@ -1,7 +1,7 @@
 package:
   name: tmux
   version: "3.5"
-  epoch: 1
+  epoch: 2
   description: Tool to control multiple terminals from a single terminal
   copyright:
     - license: ISC
@@ -30,6 +30,9 @@ environment:
       - wolfi-base
       - yaml-dev
       - zlib-dev
+  environment:
+    # TODO: drop this once tmux fixes it
+    CFLAGS: -std=gnu17
 
 pipeline:
   - uses: git-checkout