patch 8.1.1569: cannot build with signs but without diff feature

Problem:    Cannot build with signs but without diff feature.
Solution:   Move #ifdef. (Tom Ryder)
diff --git a/src/screen.c b/src/screen.c
index 016355e..914bf37 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -3051,8 +3051,8 @@
 	int		wcr_attr,
 	int		row,
 	int		startrow,
-	int		filler_lines,
-	int		filler_todo,
+	int		filler_lines UNUSED,
+	int		filler_todo UNUSED,
 	int		*c_extrap,
 	int		*c_finalp,
 	char_u		*extra,
@@ -3237,9 +3237,9 @@
     int		mb_c = 0;		/* decoded multi-byte character */
     int		mb_utf8 = FALSE;	/* screen char is UTF-8 char */
     int		u8cc[MAX_MCO];		/* composing UTF-8 chars */
+    int		filler_lines = 0;	/* nr of filler lines to be drawn */
+    int		filler_todo = 0;	/* nr of filler lines still to do + 1 */
 #ifdef FEAT_DIFF
-    int		filler_lines;		/* nr of filler lines to be drawn */
-    int		filler_todo;		/* nr of filler lines still to do + 1 */
     hlf_T	diff_hlf = (hlf_T)0;	/* type of diff highlighting */
     int		change_start = MAXCOL;	/* first col of changed area */
     int		change_end = -1;	/* last col of changed area */