discord: fixed timestamp update when seeking

When seeking in a song, `RichPresence::Seeked()` receives the new
position in microseconds and is supposed to update the RPC timestamp
with the new position in seconds. However, it actually converts the
value to milliseconds, meaning that if, for example, you seek to 0:05 in
a song, Discord will think you seeked to 83:20, or 5000 seconds from the
beginning of the song.

This commit fixes this by simply dividing the microseconds value by one
million instead of one thousand.

src/discord/richpresence.cpp

@@ -167,7 +167,7 @@ void RichPresence::Seeked(const qint64 seek_microseconds) {
 
   if (!initialized_) return;
 
-  SetTimestamp(seek_microseconds / 1000LL);
+  SetTimestamp(seek_microseconds / 1000000LL);
   SendPresenceUpdate();
 
 }