remove volatile qualification from category pointers in __locale_struct

commit 63c188ec42 missed making this
change when switching from atomics to locking for modification of the
global locale, leaving access to locale structures unnecessarily
burdened with the restrictions of volatile.

the volatile qualification was originally added in commit
56fbaa3bbe.

src/internal/libc.h

@@ -8,7 +8,7 @@
 struct __locale_map;
 
 struct __locale_struct {
-	const struct __locale_map *volatile cat[6];
+	const struct __locale_map *cat[6];
 };
 
 struct tls_module {