Fix const correctness in hv_func.h

Fix const correctness in hv_func.h

Building an XS code with -Wcast-qual yielded warnings about discarding
const qualifiers from pointer targets like:

$ printf '#include "EXTERN.h"\n#include "perl.h"\n' | gcc -Wcast-qual -I/usr/lib64/perl5/CORE -c -x c -
In file included from /usr/lib64/perl5/CORE/hv.h:629:0,
                 from /usr/lib64/perl5/CORE/perl.h:3740,
                 from <stdin>:2:
/usr/lib64/perl5/CORE/hv_func.h: In function ‘S_perl_hash_siphash_2_4’:
/usr/lib64/perl5/CORE/hv_func.h:213:17: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
   U64TYPE k0 = ((U64TYPE*)seed)[0];
                 ^

Signed-off-by: Petr Písař <ppisar@redhat.com>