connman-notify: set platforms = linux

It seems pointless to build a notification app on platforms where
connman itself is not supported.

pkgs/tools/networking/connman-notify/default.nix

···
     description = "Desktop notification integration for connman";
     homepage = https://github.com/wavexx/connman-notify;
     license = licenses.gpl2;
-    platforms = platforms.unix;
+    platforms = platforms.linux;
     maintainers = [ maintainers.romildo ];
   };
 }