build: Don't use /dev/srandom on OpenBSD.

Ported from GnuPG 1.4.

All /dev/*random devices have been equivalent since OpenBSD 4.9, on
purpose (/dev/random doesn't block). /dev/srandom has been removed in
the OpenBSD 6.3 development cycle, /dev/arandom will likely follow.

• Signed-off-by: Jeremie Courreges-Anglas <jca@wxcvbn.org>