Remove unnecessary level of indirection from dnsdecode() parameter
The `sp' pointer doesn't need to be modified in the caller of
dnsdecode().
This fixes -Wcast-qual error (`must have all intermediate pointers
const qualified to be safe') when compiled with WARNS=6.
Submitted by: Ján Sučan <sucanjan@gmail.com>
Sponsored by: Google, inc. (Google Summer of Code 2019)