Add missing mention of mandatory function OSSL_FUNC_keymgmt_has

Add missing mention of mandatory function OSSL_FUNC_keymgmt_has

The manual page provider-keymgmt.pod is missing the mention of the
required function OSSL_FUNC_keymgmt_has. The function
keymgmt_from_algorithm raise EVP_R_INVALID_PROVIDER_FUNCTIONS
if keymgmt->has == NULL

CLA: trivial
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16621)

(cherry picked from commit 56b8f434c7da35b4de16603faad4170eb1d80710)