The programs assume that including "pk.h" brings up "bignum.h". When
MBEDTLS_USE_TINYCRYPT is enabled and MBEDTLS_RSA_C is disabled, that
assumption no longer holds. Fix that by explicitly including bignum.h from the
programs that need it.
This is a bug pre-existing to this PR, fixed here to enable the inclusion of a
test with a config that happens to reveal it.