configure: Allow vnc to get disabled with --without-default-features

There's no reason why we should keep VNC enabled when the user
specified --without-default-features.

Reported-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210713093155.677589-3-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index eb85fe8..cb3856f 100644
--- a/meson.build
+++ b/meson.build
@@ -930,7 +930,7 @@
 png = not_found
 jpeg = not_found
 sasl = not_found
-if get_option('vnc').enabled()
+if not get_option('vnc').disabled()
   vnc = declare_dependency() # dummy dependency
   png = dependency('libpng', required: get_option('vnc_png'),
                    method: 'pkg-config', kwargs: static_kwargs)