meson: Display libfdt as disabled when system emulation is disabled

When configuring QEMU with --disable-system, meson keeps showing
libfdt as "auto". Mark it as disabled instead.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214183144.27402-2-f4bug@amsat.org>