Add #ifdef HB_USE_ATEXIT to fix -Wunused-function warnings

src/hb-ft.cc

diff --git a/src/hb-ft.cc b/src/hb-ft.cc
index 883f26f..7e75ba9 100644
--- a/src/hb-ft.cc
+++ b/src/hb-ft.cc
@@ -467,11 +467,13 @@
 
 static FT_Library ft_library;
 
-static inline
+#ifdef HB_USE_ATEXIT
+static
 void free_ft_library (void)
 {
   FT_Done_FreeType (ft_library);
 }
+#endif
 
 static FT_Library
 get_ft_library (void)