vim-patch:8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes vim/vim#1943)
cae92dc3d5

src/nvim/vim.h

@@ -155,6 +155,7 @@ enum {
   EXPAND_USER_ADDR_TYPE,
   EXPAND_PACKADD,
   EXPAND_MESSAGES,
+  EXPAND_MAPCLEAR,
   EXPAND_CHECKHEALTH,
 };