df22fbb7 ("qboot: update to latest submodule") updated the qboot
submodule from a5300c49 to 8ca302e8. However, qboot isn't built during
the QEMU's build process but rather is included in binary form. So
rebuild it here.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
[Rebuilt it myself for paranoia. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>