solanum: mark broken on darwin

> Solanum is developed on Linux with glibc, but is currently portable
> to most POSIX-compatible operating systems. However, this portability
> is likely to be removed unless someone is willing to maintain it. If
> you'd like to be that person, please let us know on IRC.

But darwin apparently broke:

In file included from librb/version.c:23:
librb/../include/serno.h:1:10: fatal error: 'datecode.h' file not found
    1 | #include "datecode.h"
      |          ^~~~~~~~~~~~

pkgs/by-name/so/solanum/package.nix

@@ -85,6 +85,7 @@ stdenv.mkDerivation (finalAttrs: {
   };
 
   meta = {
+    broken = stdenv.hostPlatform.isDarwin;
     description = "IRCd for unified networks";
     homepage = "https://github.com/solanum-ircd/solanum";
     license = lib.licenses.gpl2Plus;