xinput-calibrator: add formfactor to RDEPENDS

* 30xinput_calibrate.sh is calling ". /etc/formfactor/config"
  breaking Xsession for images without formfactor

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>