protonvpn-gui: Add missing dep

https://www.github.com/ProtonVPN/proton-vpn-gtk-app/blob/1d44f9aa7e4239685c1759bf9c106b3a14708e76/proton/vpn/app/gtk/__init__.py#L31

Author: Artturin

pkgs/applications/networking/protonvpn-gui/default.nix

@@ -5,6 +5,7 @@
   gobject-introspection,
   setuptools,
   wrapGAppsHook3,
+  libnotify,
   dbus-python,
   packaging,
   proton-core,
@@ -37,12 +38,16 @@ buildPythonApplication rec {
     wrapGAppsHook3
   ];
 
-  buildInputs = lib.optionals withIndicator [
-    # Adds AppIndicator3 namespace
-    libappindicator-gtk3
-    # Adds AyatanaAppIndicator3 namespace
-    libayatana-appindicator
-  ];
+  buildInputs =
+    [
+      libnotify # gir typelib is used
+    ]
+    ++ lib.optionals withIndicator [
+      # Adds AppIndicator3 namespace
+      libappindicator-gtk3
+      # Adds AyatanaAppIndicator3 namespace
+      libayatana-appindicator
+    ];
 
   build-system = [
     setuptools