tls-crypt: properly cast time_t to uint64_t

tls-crypt: properly cast time_t to uint64_t

The exact type of time_t is platform dependent and therefore
can't be assumed to be uint64_t all the time.

For example, on 32bit platforms, where time_t is defined as long
(32bit), the compiler will generate the following warning, due
to the arithmetic used in the macro:

tls_crypt.c:745:29: warning: shift count >= width of type
[-Wshift-count-overflow]

Force time_t to be parsed as uint64_t.

Reported-by: Arne Schwabe <arne@rfc2549.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20181031160124.26972-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17868.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>