cleanup: Move write_pid() to where it is being used

cleanup: Move write_pid() to where it is being used

The write_pid() function is only used in openvpn.c, so no
need to have that in the misc.[ch] mixed bag.

[on-the-fly change: Added #include "platform.h"]

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170725145718.13175-1-davids@openvpn.net>
URL: https://www.mail-archive.com/search?l=mid&q=20170725145718.13175-1-davids@openvpn.net
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit c5b12817c9aa3ae97fbdd2c2a9a9ab605087dff1)