patch 8.2.3524: GUI: ligatures are not used

Problem:    GUI: ligatures are not used.
Solution:   Add the 'guiligatures' option. (Dusan Popovic, closes #8933)
diff --git a/src/optionstr.c b/src/optionstr.c
index 06a633b..bced92d 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -1560,6 +1560,13 @@
 	redraw_gui_only = TRUE;
     }
 #endif
+# if defined(FEAT_GUI_GTK)
+    else if (varp == &p_guiligatures)
+    {
+	gui_set_ligatures();
+	redraw_gui_only = TRUE;
+    }
+# endif
 
 #ifdef CURSOR_SHAPE
     // 'guicursor'