Add SPF record type

[Th3Mouk]

Description

Add a missing const

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[styxit]

Thanks!

Even though the SPF type should not be used anymore and it's advised to use TXT instead, PowerDNS still supports this type so it makes sense that this package also supports it.

If possible SPF rules should be defined in a TXT record: https://tools.ietf.org/html/rfc7208