iwgtk: adding missing `meta.mainProgram`

pkgs/tools/networking/iwgtk/default.nix

···
     homepage = "https://github.com/j-lentz/iwgtk";
     changelog = "https://github.com/j-lentz/iwgtk/blob/v${version}/CHANGELOG";
     license = licenses.gpl3Only;
+    maintainers = with maintainers; [ figsoda ];
     platforms = platforms.linux;
-    maintainers = with maintainers; [ figsoda ];
+    mainProgram = "iwgtk";
   };
 }