banned.h: mark non-reentrant gmtime, etc as banned

The traditional gmtime(), localtime(), ctime(), and asctime() functions return pointers to shared storage. This means they're not thread-safe, and they also run the risk of somebody holding onto the result across multiple calls (where each call invalidates the previous result). All callers should be using their reentrant counterparts. Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>