patch 8.1.1256: cannot navigate through errors relative to the cursor

Problem:    Cannot navigate through errors relative to the cursor.
Solution:   Add :cabove, :cbelow, :labove and :lbelow. (Yegappan Lakshmanan,
            closes #4316)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index b317f53..db9bd06 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -56,6 +56,7 @@
 # define ex_cbuffer		ex_ni
 # define ex_cc			ex_ni
 # define ex_cnext		ex_ni
+# define ex_cbelow		ex_ni
 # define ex_cfile		ex_ni
 # define qf_list		ex_ni
 # define qf_age			ex_ni