Add guard around free_static_C_locale

Add HB_USE_ATEXIT guard around free_static_C_locale missed by previous
commit.
diff --git a/src/hb-common.cc b/src/hb-common.cc
index 22dd52f..614b71b 100644
--- a/src/hb-common.cc
+++ b/src/hb-common.cc
@@ -731,8 +731,9 @@
 
 #ifdef USE_XLOCALE
 
-
+#ifdef HB_USE_ATEXIT
 static void free_static_C_locale (void);
+#endif
 
 static struct hb_C_locale_lazy_loader_t : hb_lazy_loader_t<hb_remove_ptr_t<HB_LOCALE_T>::value,
 							  hb_C_locale_lazy_loader_t>