meson: Fix targetos match for illumos and Solaris.

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>
(cherry picked from commit fb0a8b0e238277296907ffe765bf76874cfc1df6)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: omit net/meson.build change before v8.1.0-279-g73258b3864, adjust context befor v8.1.0-288-g2fc36530de)