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

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

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

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

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