send-email: make_message_id use "require" instead of "use"

send-email: make_message_id use "require" instead of "use"

Change the use of Sys::Hostname from a "use" to a "require". The
former happens in an implicit BEGIN block and is thus immune from the
if block it's contained in, so it's always loaded.

This should speed up the invocation of git-send-email by a few
milliseconds.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Reviewed-by: Jeff King <redacted>
>
Signed-off-by: Junio C Hamano <redacted>