Fix#10990.
A warning is triggered when snprintf is used because it is being
declared / configured incorrectly. The MBEDTLS configuration is
defaulting to disabling all of LIBC, and then explicitly enabling the
LIBC functions in use, this has been done for printf with
MBEDTLS_PLATFORM_STD_PRINTF, but not for
MBEDTLS_PLATFORM_STD_SNPRINTF.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Sherry Zhang