librime: set darwin as support platforms

pkgs/development/libraries/librime/default.nix

···
     description = "Rime Input Method Engine, the core library";
     license     = licenses.bsd3;
     maintainers = with maintainers; [ vonfry ];
-    platforms   = platforms.linux;
+    platforms   = platforms.linux ++ platforms.darwin;
   };
 }