samples: nrf52: power_mgr: Fix build with prj_tickless.conf config

prj_tickless.conf lacks the CONFIG_PM_CONTROL_OS=y option which
results in the sample failing to build with this config as it leaves
sys_pm_ctrl_disable_state() and sys_pm_ctrl_enable_state()
undefined.

Add the relevant options from prj.conf to fix the build.

Tested on nrf52840_pca10056.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>