Simplify the guards on MBEDTLS_ERROR_C and MBEDTLS_ERROR_STRERROR_DUMMY.
No longer include superfluous headers and definition: string.h and
platform.h are only needed for MBEDTLS_ERROR_C; time_t is not needed
at all.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>