Only use Qt's Deselect shortcut on Linux

Because on all other platforms it's apparently undefined, leading to
Emscripten not having it assigned. It was already set to Ctrl+Shift+A on
Windows and macOS, now it's also set tha way on Android and Emscripten,
basically.

src/desktop/mainwindow.cpp

@@ -5154,8 +5154,8 @@ void MainWindow::setupActions()
 	//
 	// Select menu
 	//
-	// Deselect is not defined on macOS and Windows.
-#if defined(Q_OS_MACOS) || defined(Q_OS_WIN)
+	// Deselect is not defined on most systems.
+#ifndef Q_OS_LINUX
 #	define DESELECT_SHORTCUT "Ctrl+Shift+A"
 #else
 #	define DESELECT_SHORTCUT QKeySequence::Deselect