patch 8.2.3510: changes are only detected with one second accuracy

Problem:    Changes are only detected with one second accuracy.
Solution:   Use the nanosecond time if possible.  (Leah Neukirchen,
            closes #8873, closes #8875)
diff --git a/src/structs.h b/src/structs.h
index 0715686..7dac95b 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2724,7 +2724,9 @@
     wininfo_T	*b_wininfo;	// list of last used info for each window
 
     long	b_mtime;	// last change time of original file
+    long	b_mtime_ns;	// nanoseconds of last change time
     long	b_mtime_read;	// last change time when reading
+    long	b_mtime_read_ns;  // nanoseconds of last read time
     off_T	b_orig_size;	// size of original file in bytes
     int		b_orig_mode;	// mode of original file
 #ifdef FEAT_VIMINFO