event/Thread: reduce the RTIO timer slack to 10us

MPD's default is 100ms, which is too long for the real-time I/O
thread.  The OutputThread has 100us, but the real-time I/O thread
might have tighter deadlines.

This change has currently no effect (I believe), because nobody uses
timers on the RTIO thread.

src/event/Thread.cxx

@@ -20,6 +20,7 @@
 #include "config.h"
 #include "Thread.hxx"
 #include "thread/Name.hxx"
+#include "thread/Slack.hxx"
 #include "thread/Util.hxx"
 #include "Log.hxx"
 
@@ -46,6 +47,8 @@ EventThread::Run() noexcept
 	SetThreadName(realtime ? "rtio" : "io");
 
 	if (realtime) {
+		SetThreadTimerSlackUS(10);
+
 		try {
 			SetThreadRealtime();
 		} catch (...) {