patch 9.0.0656: cannot specify another character to use instead of '@'

Problem:    Cannot specify another character to use instead of '@' at the end
            of the window.
Solution:   Add "lastline" to 'fillchars'. (Martin Tournoij, closes #11264,
            closes #10963)

src/structs.h

@@ -3519,6 +3519,7 @@ typedef struct
     int	foldsep;
     int	diff;
     int	eob;
+    int	lastline;
 } fill_chars_T;
 
 /*