pfixtools: set -Wno-error=unused-result

hardening enables further warnings breaking the build

+2
+2
pkgs/servers/mail/postfix/pfixtools.nix
··· 38 --replace /bin/bash ${bash}/bin/bash; 39 ''; 40 41 makeFlags = "DESTDIR=$(out) prefix="; 42 43 meta = {
··· 38 --replace /bin/bash ${bash}/bin/bash; 39 ''; 40 41 + NIX_CFLAGS_COMPILE = "-Wno-error=unused-result"; 42 + 43 makeFlags = "DESTDIR=$(out) prefix="; 44 45 meta = {