dhcp: fixup build with gcc-7 (ignore warning)

Upgrade to latest release wouldn't solve this one,
and it would conflict with both of our patches,
so I decided not to spend much time on this for now.

pkgs/tools/networking/dhcp/default.nix

···
     "--localstatedir=/var"
   ] ++ stdenv.lib.optionals (openldap != null) [ "--with-ldap" "--with-ldapcrypto" ];
 
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=pointer-compare" ];
+
   installFlags = [ "DESTDIR=\${out}" ];
 
   postInstall =