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>

stubs/meson.build

@@ -15,7 +15,9 @@ stub_ss.add(files('fdset.c'))
 stub_ss.add(files('fw_cfg.c'))
 stub_ss.add(files('gdbstub.c'))
 stub_ss.add(files('get-vm-name.c'))
-stub_ss.add(when: 'CONFIG_LINUX_IO_URING', if_true: files('io_uring.c'))
+if linux_io_uring.found()
+  stub_ss.add(files('io_uring.c'))
+endif
 stub_ss.add(files('iothread-lock.c'))
 stub_ss.add(files('isa-bus.c'))
 stub_ss.add(files('is-daemonized.c'))