Fix notifications hiding on Linux.

Regression was introduced in 3372dfcd3e.

Only when platform-specific code can give us the global time of the
last user input event we rely on idle time for notifications hiding.

Fixes #5791.

Telegram/SourceFiles/platform/linux/specific_linux.h

@@ -33,7 +33,9 @@ inline void ReInitOnTopPanel(QWidget *panel) {
 
 QString CurrentExecutablePath(int argc, char *argv[]);
 
-crl::time LastUserInputTime();
+inline std::optional<crl::time> LastUserInputTime() {
+	return std::nullopt;
+}
 
 } // namespace Platform
 
@@ -51,8 +53,6 @@ void psWriteDump();
 
 void psDeleteDir(const QString &dir);
 
-bool psIdleSupported();
-
 QStringList psInitLogs();
 void psClearInitLogs();