patch 8.0.0334: can't access b:changedtick from a dict reference

Problem:    Can't access b:changedtick from a dict reference.
Solution:   Make changedtick a member of the b: dict. (inspired by neovim
            #6112)

src/structs.h

@@ -1916,7 +1916,9 @@ struct file_buffer
 
     int		b_changed;	/* 'modified': Set to TRUE if something in the
 				   file has been changed and not written out. */
-    int		b_changedtick;	/* incremented for each change, also for undo */
+    varnumber_T	*b_changedtick;	/* points into b:changedtick or b_ct_val;
+				   incremented for each change, also for undo */
+    varnumber_T b_ct_val;	/* fallback for b:changedtick */
 
     int		b_saving;	/* Set to TRUE if we are in the middle of
 				   saving the buffer. */