configure: accept --enable-slirp

Meson understands -Dslirp=enabled, so there is no reason not to
accept the configure option as well.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed