sysutils/openzfs-kmod: fix build on powerpc64*

/wrkdirs/usr/ports/sysutils/openzfs-kmod/work/zfs-2d5622f5b/module/icp/algs/blake3/blake3_x86-64.c:78:28: error: implicit declaration of function 'zfs_vsx_available' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

return (kfpu_allowed() && zfs_vsx_available());
                          ^

/wrkdirs/usr/ports/sysutils/openzfs-kmod/work/zfs-2d5622f5b/module/icp/algs/blake3/blake3_x86-64.c:144:28: error: implicit declaration of function 'zfs_vsx_available' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

return (kfpu_allowed() && zfs_vsx_available());