Build fix for databases/openldap-smbk5pwd

From upstream ITS #8885

Add a configure test for hdb_generate_key_set_password() prototype

contrib/slapd-modules/smbk5pwd uses hdb_generate_key_set_password() from
Heimdal, which was shortly turned from a 5 arguments function to a 7 arguments
function before the prototype change was rolled back to address API
incompatibility.

Unfortunately, the 7 arguments hdb_generate_key_set_password() made it into
released NetBSD 8.0, causing a build break in contrib/slapd-modules/smbk5pwd.

This change adds a configure test for 7 arguments prototype so that
contrib/slapd-modules/smbk5pwd build again on NetBSD 8.0, and other OS that
would include the 7 arguments hdb_generate_key_set_password().

(manu)