patch 8.1.0287: MAX is not defined everywhere

Problem:    MAX is not defined everywhere.
Solution:   Define MAX where needed.

src/ex_getln.c

@@ -13,6 +13,10 @@
 
 #include "vim.h"
 
+#ifndef MAX
+# define MAX(x,y) ((x) > (y) ? (x) : (y))
+#endif
+
 /*
  * Variables shared between getcmdline(), redrawcmdline() and others.
  * These need to be saved when using CTRL-R |, that's why they are in a