_2bwm: fix build with GCC 15

Pass -std=gnu17 to work around incompatible-pointer-types errors.
The upstream code declares callback functions with empty parameter
lists, which GCC 15 (defaulting to gnu23) now rejects.

Ref: #475479

pkgs/applications/window-managers/2bwm/default.nix

@@ -25,6 +25,8 @@ stdenv.mkDerivation rec {
   # Allow users set their own list of patches
   inherit patches;
 
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   buildInputs = [
     libxcb
     libxcb-keysyms