Turn off VMS C's info about unsupported pragmas

Turn off VMS C's info about unsupported pragmas

VMS C can be notoriously informative about certain things, such as
unsupported pragmas.  The case here is that it doesn't support
"#pragma once", and since we use those quite a lot, that's a lot of
repeated information.  We simply turn that warning off.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15317)