Fix tests build, provide mocks for libinput functions

autotests/libinput/mock_libinput.cpp

@@ -922,3 +922,17 @@ libinput_device_tablet_pad_get_num_buttons(struct libinput_device *device)
 {
     return device->buttonCount;
 }
+
+struct libinput_device_group *
+libinput_device_get_device_group(struct libinput_device *device)
+{
+    Q_UNUSED(device);
+    return nullptr;
+}
+
+void *
+libinput_device_group_get_user_data(struct libinput_device_group *group)
+{
+    Q_UNUSED(group);
+    return nullptr;
+}