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)
diff --git a/src/screen.c b/src/screen.c
index ebb5ffc..e5fd4bf 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2511,7 +2511,7 @@
 		    else
 			force_next = FALSE;
 		}
-#endif
+#endif // FEAT_GUI || defined(UNIX)
 		ScreenLines[off] = c;
 		if (enc_utf8)
 		{
@@ -4943,6 +4943,7 @@
 	{&fill_chars.foldsep,	"foldsep"},
 	{&fill_chars.diff,	"diff"},
 	{&fill_chars.eob,	"eob"},
+	{&fill_chars.lastline,	"lastline"},
     };
 
     static lcs_chars_T lcs_chars;
@@ -5022,6 +5023,7 @@
 		fill_chars.foldsep = '|';
 		fill_chars.diff = '-';
 		fill_chars.eob = '~';
+		fill_chars.lastline = '@';
 	    }
 	}
 	p = value;