libmilter: set MILTER_SOVER

MILTER_SOVER is used as an environment variable during the build, but never set anywhere
set to so version 1 as currently used by Ubuntu

pkgs/development/libraries/libmilter/default.nix

@@ -28,6 +28,7 @@ stdenv.mkDerivation rec {
       define(\`confLIBGRP', \`root')
       APPENDDEF(\`confENVDEF', \`-DNETINET6')
     EOF
+    export MILTER_SOVER=1
     sh Build -f ./a.m4
   '';