modules: mbedtls: fix ITS configuration on POSIX architecture

Make sure that `MBEDTLS_PSA_ITS_FILE_C` is not defined when
`CONFIG_SECURE_STORAGE` is enabled as the former takes precedence over
the latter in Mbed TLS (`psa_crypto_storage.c`).

This fixes usage of the secure storage subsystem by Mbed TLS on platforms
such as `native_sim`.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>