Work around RHEL8 pooping the bed in OpenSSL's headers

[rgacogne]

Short description

The openssl/kdf.h header on RHEL8 is invalid because someone backported a work-in-progress feature to an older OpenSSL branch and did not bother to backport the fixes that were added later.

Red Hat declined to fix their mess and helpfully suggested we do the work instead in https://bugzilla.redhat.com/show_bug.cgi?id=2215856

This will need to be backported to all the branches we want to support on RHEL 8 and co.

Fixes #12926:

• before checking whether EVP_PKEY_CTX_set1_scrypt_salt is declared... no
• after checking whether EVP_PKEY_CTX_set1_scrypt_salt is declared... yes and it actually compiles

# pdns/pdnsutil hash-password
Warning: unable to read configuration file '/usr/local/etc/pdns.conf': No such file or directory
$scrypt$ln=10,p=1,r=8$Vtrf782VeuoQNY9VE7mvUA==$KFn4DGLV0SV4O7Pxunb38MBHr3uCf5SXtfzsf30aeBc=

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[Habbie]

Fixes #12613

should be #12926

[rgacogne]

Fixes #12613

should be #12926

Oops, correct! Sorry but I'm not going to fix it until there is a CVE assigned to it!