maddy: fix build with clang

pkgs/servers/maddy/default.nix

@@ -1,4 +1,13 @@
-{ lib, buildGoModule, fetchFromGitHub, pam, coreutils, installShellFiles, scdoc, nixosTests }:
+{ lib
+, stdenv
+, buildGoModule
+, fetchFromGitHub
+, pam
+, coreutils
+, installShellFiles
+, scdoc
+, nixosTests
+}:
 
 buildGoModule rec {
   pname = "maddy";
@@ -43,6 +52,8 @@ buildGoModule rec {
       --replace "/bin/kill" "${coreutils}/bin/kill"
   '';
 
+  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=strict-prototypes";
+
   passthru.tests.nixos = nixosTests.maddy;
 
   meta = with lib; {