net: lib: sockets_tls: Include zephyr_mbedtls_priv.h conditionally

This is a follow-up to commit a418ad4b.

Since the path to zephyr_mbedtls_priv.h is added to include directories
only when CONFIG_MBEDTLS_BUILTIN is enabled, the inclusion of the file
needs to be done under the same condition. Otherwise, an error occurs
when socket_tls.c is compiled without CONFIG_MBEDTLS_BUILTIN.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>