librsb: Mark broken on Darwin

Builds haven't succeeded on Hydra in close to a year.

pkgs/development/libraries/librsb/default.nix

@@ -82,5 +82,7 @@ stdenv.mkDerivation rec {
     license = with licenses; [ lgpl3Plus ];
     maintainers = with maintainers; [ KarlJoad ];
     platforms = platforms.all;
+    # ./rsb_common.h:56:10: fatal error: 'omp.h' file not found
+    broken = stdenv.isDarwin;
   };
 }