package/screen: add /usr/bin/screen to /etc/shells

package/screen: add /usr/bin/screen to /etc/shells

When screen is selected, /usr/bin/screen is not added to /etc/shells
(see man shells). So, login tools like dropbear reject the ssh
connections for users using screen as shell in /etc/passwd.

buildroot authpriv.warn dropbear[853]: User 'kubu' has invalid shell, rejected

While at it, drop an empty line between an existing hook definition
and its registration, to be consistent with the coding style used in
the rest of Buildroot.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>