patch 9.0.0036: 'fillchars' cannot have window-local values

Problem:    'fillchars' cannot have window-local values.
Solution:   Make 'fillchars' global-local. (closes #5206)
diff --git a/src/mouse.c b/src/mouse.c
index 2297f81..b011609 100644
--- a/src/mouse.c
+++ b/src/mouse.c
@@ -2018,7 +2018,7 @@
 	count |= CURSOR_MOVED;		// Cursor has moved
 
 # ifdef FEAT_FOLDING
-    if (mouse_char == fill_foldclosed)
+    if (mouse_char == curwin->w_fill_chars.foldclosed)
 	count |= MOUSE_FOLD_OPEN;
     else if (mouse_char != ' ')
 	count |= MOUSE_FOLD_CLOSE;