openssl: Add <sys/random.h> include for getrandom()

GCC 14 (but not earlier versions) warns about a missing prototype
for getrandom(). Include <sys/random.h> explicitly to bring in the
prototype rather than depending on a nested include. While here,
stop defining sysctl_random() since it is no longer used.

Reviewed by: brooks
Fixes: 838b6caababb openssl: use getrandom(2) instead of probing for getentropy(2)
Differential Revision: https://reviews.freebsd.org/D45995