tty: Move session_of_pgrp() and make static

tiocspgrp() is the lone caller of session_of_pgrp(); relocate and
limit to file scope.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>