patch 8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes #1943)
diff --git a/src/vim.h b/src/vim.h
index 2562d9e..d7d3556 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -808,6 +808,7 @@
 #define EXPAND_USER_ADDR_TYPE	44
 #define EXPAND_PACKADD		45
 #define EXPAND_MESSAGES		46
+#define EXPAND_MAPCLEAR		47
 
 /* Values for exmode_active (0 is no exmode) */
 #define EXMODE_NORMAL		1