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/gui.h b/src/gui.h
index b7b526d..9806c83 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -409,6 +409,9 @@
     char_u	*browse_fname;	    // file name from filedlg
 
     guint32	event_time;
+
+    char_u ligatures_map[256];	    // ascii map for characters 0-255, value is
+				    // 1 if in 'guiligatures'
 #endif	// FEAT_GUI_GTK
 
 #if defined(FEAT_GUI_TABLINE) \