secure storage: Add missing include for mbedtls_platform_zeroize

-Adding explicit include for mbedtls/platform_util.h to get acces
 to mbedtls_platform_zeroize in ITS. Somehow not visibile in Zephyr but
 it caused build issues in nRF Connect SDK.

Signed-off-by: Frank Audun Kvamtrø <frank.kvamtro@nordicsemi.no>