meson: Don't even detect posix_madvise() on Darwin

meson: Don't even detect posix_madvise() on Darwin

On Darwin, posix_madvise() has the same return semantics as plain
madvise() [1]. That's not really what our usage expects.
Fortunately, madvise() is available and preferred anyways so we
may stop detecting posix_madvise() on Darwin.

1: https://opensource.apple.com/source/xnu/xnu-7195.81.3/bsd/man/man2/madvise.2.auto.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-ID: <00f71753bdeb8c0f049fda05fb63b84bb5502fb3.1717584048.git.mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>