[gi/ft] Silence warnings

gi doesn't understand that FT_Face is a pointer, and not bare struct.
So it skips these APIs anyway. Mark skip to silense the warning.

../src/hb-ft.cc:242: Warning: HarfBuzz: hb_ft_font_get_face: return value: Invalid non-constant return of bare structure or union; register as boxed type or (skip)
../src/hb-ft.cc:264: Warning: HarfBuzz: hb_ft_font_lock_face: return value: Invalid non-constant return of bare structure or union; register as boxed type or (skip)

src/hb-ft.cc

@@ -233,7 +233,7 @@ hb_ft_font_get_load_flags (hb_font_t *font)
 }
 
 /**
- * hb_ft_font_get_face:
+ * hb_ft_font_get_face: (skip)
  * @font: #hb_font_t to work upon
  *
  * Fetches the FT_Face associated with the specified #hb_font_t
@@ -255,7 +255,7 @@ hb_ft_font_get_face (hb_font_t *font)
 }
 
 /**
- * hb_ft_font_lock_face:
+ * hb_ft_font_lock_face: (skip)
  * @font: #hb_font_t to work upon
  *
  * Gets the FT_Face associated with @font, This face will be kept around until