meson.build: Make keyutils independent from keyring

Commit 0db0fbb5 ("Add conditional dependency for libkeyutils")
tried to provide a possibility for the user to disable keyutils
if not required by makeing it depend on the keyring feature. This
looked reasonable at a first glance (the unit test in tests/unit/
needs both), but the condition in meson.build fails if the feature
is meant to be detected automatically, and there is also another
spot in backends/meson.build where keyutils is used independently
from keyring. So let's remove the dependency on keyring again and
introduce a proper meson build option instead.

Cc: qemu-stable@nongnu.org
Fixes: 0db0fbb5 ("Add conditional dependency for libkeyutils")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1842


Message-ID: <20230824094208.255279-1-thuth@redhat.com>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>