pk: mbedtls_pk_can_do_ext() should be guarded by CRYPTO_CLIENT instead
of CRYPTO_C.
test: since the functions using opaque keys are now guarded by CRYPTO_C
and since CRYPTO_CLIENT is automatically enabled as soon as
CRYPTO_C is, then CRYPTO_CLIENT guards can be removed.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>