Fix IP_MTU_DISCOVER typo

The code in bss_dgram.c checks if IP_MTUDISCOVER is defined, where it
should test for IP_MTU_DISCOVER

RT#3888

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 0baaff1a)