graphics/libjxl: fails to build on powerpc64

third_party/highway/hwy/nanobenchmark.cc:50:10: fatal error: 'sys/platform/ppc.h' file not found
#include <sys/platform/ppc.h> // NOLINT __ppc_get_timebase_freq

^~~~~~~~~~~~~~~~~~~~

Reported by: pkg-fallout

(cherry picked from commit c350bf15e24eb44eb5576db28322d78bee7ab23b)