MBEDTLS_HAVE_TIME is documented as: "System has time.h and time()."
If that is not defined, do not attempt to include time.h.
A particular problem is platform-time.h, which should only be included if
MBEDTLS_HAVE_TIME is defined, which makes everything messier. Maybe it
should be refactored to have the check inside the header.
Signed-off-by: Daniel Axtens <dja@axtens.net>