[gtksourceview] utils: add missing FONT_STYLE define

[Christian Hergert <chergert src gnome org>]

commit 92c36bfe54c8b2372729106dd303dee62bbcbfe9
Author: Christian Hergert <chergert redhat com>
Date:   Mon Oct 19 10:23:00 2020 -0700

    utils: add missing FONT_STYLE define
    
    Fixes !168

 gtksourceview/gtksourceutils.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtksourceview/gtksourceutils.c b/gtksourceview/gtksourceutils.c
index ce3226dc..72e00616 100644
--- a/gtksourceview/gtksourceutils.c
+++ b/gtksourceview/gtksourceutils.c
@@ -322,6 +322,7 @@ _gtk_source_utils_string_to_int (const gchar *str)
 #define FONT_VARIANT "font-variant"
 #define FONT_STRETCH "font-stretch"
 #define FONT_WEIGHT  "font-weight"
+#define FONT_STYLE   "font-style"
 #define FONT_SIZE    "font-size"
 
 gchar *