meson: fix logic for gnutls check

meson: fix logic for gnutls check

The logic before was

	if not get_option('gnutls').auto() or have_system

Which is equivalent to

	if get_option('gnutls').enabled() or get_option('gnutls').disabled() or have_system

This means that the check for gnutls is performed even if gnutls is
disabled, which means that the build system will insist on having
libtasn1 if gnutls is found, even if gnutls support is disabled.

When gnutls is disabled, the check for gnutls shouldn't be performed,
to ensure that further build system logic (like the check for
libtasn1) doesn't make decisions based on the presence of gnutls,
rather than the gnutls option.

After making this change, I can successfully ./configure --disable-gnutls
on my system with gnutls installed, but not libtasn1.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Message-Id: <20210806144947.321647-1-hi@alyssa.is>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>