meson: fix condition for io_uring stubs

meson: fix condition for io_uring stubs

CONFIG_LINUX_IO_URING is not included in config-host.mak and therefore is
not usable in "when" clauses.  Check the availability of the library,
which matches the condition for the non-stubbed version block/io_uring.c.

At this point, the difference between libraries that have config-host.mak
entries and those that do not is quite confusing.  The remaining ~dozen
should be converted in 6.2.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210712151810.508249-1-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>