firebird: mark as broken

Hasn't been updated since 2017 and breaks when building with a more
recent gcc (gcc5 needs to be removed as it breaks with glibc 2.30). I
tried to update this to a more recent version which didn't work either,
but as this is pretty old I'm not even sure if this package is used atm.

pkgs/servers/firebird/default.nix

@@ -85,6 +85,7 @@ stdenv.mkDerivation rec {
     license = ["IDPL" "Interbase-1.0"];
     maintainers = [stdenv.lib.maintainers.marcweber];
     platforms = stdenv.lib.platforms.linux;
+    broken = true;
   };
 
 }