cmake: Suppress the stderr from 'git describe' on successfull runs

A confusing warning has popped up since 3.13. It can be observed as
so:

Zephyr version: 1.13.99
Parsing Kconfig tree in /home/sebo/zephyr/Kconfig
Merging /home/sebo/zephyr/samples/hello_world/prj.conf
warning: tag 'zephyr-v1.13.0' is really 'v1.13.0' here
-- Generating zephyr/include/generated/generated_dts_board.h

This warning is more confusing than useful so we suppress stderr from
'git describe' when the command runs successfully.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>