meson: Build qemu-nbd on macOS again

Before switching to the meson build system, we used to compile qemu-nbd
for macOS, too, which is especially important for running the iotests
there. Commit b7c70bf disabled it by accident, since it did not take
into consideration that the $bsd variable in the configure script was
also set to "yes" on macOS. Fix it by enabling qemu-nbd on all systems
but Windows now instead (which was likely the original intention of the
old code in the configure script).

Fixes: b7c70bf ("meson: qemu-{img,io,nbd}")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -1081,7 +1081,7 @@ if have_tools
   qemu_io = executable('qemu-io', files('qemu-io.c'),
              dependencies: [block, qemuutil], install: true)
   qemu_block_tools += [qemu_img, qemu_io]
-  if targetos == 'linux' or targetos == 'sunos' or targetos.endswith('bsd')
+  if targetos != 'windows'
     qemu_nbd = executable('qemu-nbd', files('qemu-nbd.c'),
                dependencies: [block, qemuutil], install: true)
     qemu_block_tools += [qemu_nbd]