multimon-ng: fix license, add maintainer

pkgs/applications/radio/multimon-ng/default.nix

···
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     '';
     homepage = "https://github.com/EliasOenal/multimon-ng";
-    license = licenses.gpl2;
+    license = licenses.gpl2Only;
     platforms = platforms.linux;
+    maintainers = [ maintainers.markuskowa ];
   };
 }