programs/test: fix build without MBEDTLS_HAVE_TIME
programs/test: fix build without MBEDTLS_HAVE_TIME

Allow programs/test/udp_proxy.c to build when MBEDTLS_HAVE_TIME is
not defined. In this case, do not attempt to seed the pseudo-random
number generator used to sometimes produce corrupt packets and other
erroneous data.

Signed-off-by: David Horstmann <0386b5e8f8e5ea8a58d40026a5369df20e1c750d@arm.com>

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls