dso: eliminate VMS code on non-VMS systems

Even though the meat of dso_vms.c is compiled out on non-VMS builds,
the (pre-)compiler still traverses some of the macro handling. This
trips up at least one non-VMS build configuration, so this commit
makes the skip-VMS case more robust.

Signed-off-by: Geoff Thorpe <geoff@openssl.org>