onepad: reset the screen saver every 4096 Vsync

I didn't put the code in PCSX2 because it requires the display pointer.
And I'm afraid of multithread issue.

Related to issue #942

plugins/onepad/Linux/linux.cpp

@@ -160,6 +160,16 @@ void PollForJoystickInput(int cpad)
 
 EXPORT_C_(void) PADupdate(int pad)
 {
+	// Gamepad inputs don't count as an activity. Therefore screensaver will
+	// be fired after a couple of minute.
+	// Emulate an user activity
+	static int count = 0;
+	if ((count & 0xFFF) == 0) {
+		// 1 call every 4096 Vsync is enough
+		count++;
+		XResetScreenSaver(GSdsp);
+	}
+
 	// Actually PADupdate is always call with pad == 0. So you need to update both
 	// pads -- Gregory
 	for (int cpad = 0; cpad < 2; cpad++) {