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)
diff --git a/src/globals.h b/src/globals.h
index f3e5155..831cbf1 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1088,7 +1088,7 @@
 			= INIT_POS_T(0, 0, 0)
 # endif
 			;
-EXTERN int	last_changedtick INIT(= 0);   /* for TextChanged event */
+EXTERN varnumber_T last_changedtick INIT(= 0);   /* for TextChanged event */
 EXTERN buf_T	*last_changedtick_buf INIT(= NULL);
 #endif