Fix build without MBEDTLS_FS_IO

Fix missing definition of mbedtls_zeroize when MBEDTLS_FS_IO is
disabled in the configuration.

Introduced by e2985323947112a75c7c512ad062aa94da24cdbe
    Merge remote-tracking branch 'upstream-public/pr/1113' into mbedtls-1.3
1 file changed