devel/xxhash: fix static_assert to _Static_assert
This fixes C11+ users of xxhash that do not #include <assert.h>
PR: 265819
Approved by: portmgr@ (blanket, one-line fix to unbreak py-borgbackup)
MFH: 2022Q3
(cherry picked from commit 8a4d5e26a9721eae2deba5fef794a2e593170658)