mbedtls_vsnprintf replacement works like mbedtls_snprintf replacement, so
copy the requirements for MBEDTLS_PLATFORM_VSNPRINTF_ALT.
(MBEDTLS_PLATFORM_xxx_MACRO shouldn't require MBEDTLS_PLATFORM_C, but that's
a separate preexisting problem which I do not try address at this time.)
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>