Add braces for subobject initializer in _hb_font_funcs_nil

To avoid triggering -Wmissing-braces; see https://bugzilla.mozilla.org/show_bug.cgi?id=1226175#c8

src/hb-font.cc

@@ -297,10 +297,12 @@ static const hb_font_funcs_t _hb_font_funcs_nil = {
 #undef HB_FONT_FUNC_IMPLEMENT
   },
   {
+    {
 #define HB_FONT_FUNC_IMPLEMENT(name) hb_font_get_##name##_nil,
       HB_FONT_FUNCS_IMPLEMENT_CALLBACKS
 #undef HB_FONT_FUNC_IMPLEMENT
     }
+  }
 };
 static const hb_font_funcs_t _hb_font_funcs_parent = {
   HB_OBJECT_HEADER_STATIC,
@@ -318,10 +320,12 @@ static const hb_font_funcs_t _hb_font_funcs_parent = {
 #undef HB_FONT_FUNC_IMPLEMENT
   },
   {
+    {
 #define HB_FONT_FUNC_IMPLEMENT(name) hb_font_get_##name##_parent,
       HB_FONT_FUNCS_IMPLEMENT_CALLBACKS
 #undef HB_FONT_FUNC_IMPLEMENT
     }
+  }
 };