postfix: fix build with gcc15

- add "-std=gnu17" to `ccargs`

Fixes build failure with gcc15:
```
In file included from flush_clnt.c:96:
./mail_params.h:17:13: error: 'bool' cannot be defined via 'typedef'
   17 | typedef int bool;
      |             ^~~~
./mail_params.h:17:13: note: 'bool' is a keyword with '-std=c23' onwards
./mail_params.h:17:1: warning: useless type name in empty declaration
   17 | typedef int bool;
      | ^~~~~~~
```

pkgs/by-name/po/postfix/package.nix

@@ -38,6 +38,9 @@ let
       "-DUSE_CYRUS_SASL"
       "-I${cyrus_sasl.dev}/include/sasl"
       "-DHAS_DB_BYPASS_MAKEDEFS_CHECK"
+      # Fix build with gcc15, no upstream fix for stable releases:
+      # https://www.mail-archive.com/postfix-devel@postfix.org/msg01270.html
+      "-std=gnu17"
     ]
     ++ lib.optional withPgSQL "-DHAS_PGSQL"
     ++ lib.optionals withMySQL [