fix build/clear notifications on exit on OpenBSD

CMake builds ManagerLinux.cpp on `!MAC && !WIN`, that is BSD and Linux.
Fix the Linux-only assumption in the shared header.

Qt's `Q_OS_UNIX` is defined on Linux, BSDs and macOS alike.

src/ChatPage.cpp

@@ -1661,7 +1661,7 @@ ChatPage::isRoomActive(const QString &room_id)
 void
 ChatPage::removeAllNotifications()
 {
-#if defined(Q_OS_LINUX)
+#if defined(Q_OS_UNIX) && !defined(Q_OS_MACOS)
     notificationsManager->closeAllNotifications();
 #endif
 }

src/notifications/Manager.h

@@ -53,7 +53,7 @@ public slots:
 #if defined(NHEKO_DBUS_SYS)
 public:
     void closeNotifications(QString roomId);
-#if defined(Q_OS_LINUX)
+#if defined(Q_OS_UNIX) && !defined(Q_OS_MACOS)
     void closeAllNotifications();
 #endif