meson: Fix targetos match for illumos and Solaris.

qemu 8.1.0 breaks on illumos platforms due to _XOPEN_SOURCE and others no longer being set correctly, leading to breakage such as:

  https://us-central.manta.mnx.io/pkgsrc/public/reports/trunk/tools/20230908.1404/qemu-8.1.0/build.log



This is a result of meson conversion which incorrectly matches against 'solaris' instead of 'sunos' for uname.

First time submitting a patch here, hope I did it correctly.  Thanks.

Signed-off-by: Jonathan Perkin <jonathan@perkin.org.uk>
Message-ID: <ZPtdxtum9UVPy58J@perkin.org.uk>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>