drivers: watchdog_shell: fix compiler error

Fix wdt_shell.c:54:13: error: implicit declaration of function 'strtoul'

Tested with west build -p -b nrf52dk_nrf52832 samples/drivers/watchdog
-DCONFIG_SHELL=y.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
1 file changed