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/proto/quickfix.pro b/src/proto/quickfix.pro
index c8d5956..1ae339e 100644
--- a/src/proto/quickfix.pro
+++ b/src/proto/quickfix.pro
@@ -23,6 +23,7 @@
 int qf_get_cur_valid_idx(exarg_T *eap);
 void ex_cc(exarg_T *eap);
 void ex_cnext(exarg_T *eap);
+void ex_cbelow(exarg_T *eap);
 void ex_cfile(exarg_T *eap);
 void ex_vimgrep(exarg_T *eap);
 int get_errorlist(qf_info_T *qi_arg, win_T *wp, int qf_idx, list_T *list);