Tracker: fix thumbnail caching

system_time returns the number of microseconds since booting. This is
not what's needed for the thumbnail generation timestamp attribute, as a
result the attribute was always considered out of date and the stored
attribute would never be used.

Change-Id: I3728077c484f341b765700532d3f986e64f165ad
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5767
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: John Scipione <jscipione@gmail.com>

src/kits/tracker/Thumbnails.cpp

@@ -224,7 +224,7 @@ GenerateThumbnailJob::Execute()
 			thumbnailWritten = (status == B_OK);
 
 			// write thumbnail creation time into an attribute
-			bigtime_t created = system_time();
+			bigtime_t created = real_time_clock_usecs();
 			fFile->WriteAttr(kAttrThumbnailCreationTime, B_TIME_TYPE,
 				0, &created, sizeof(bigtime_t));
 		}