games/freecell-solver: fix build on powerpc64le

Error:
/wrkdirs/usr/ports/games/freecell-solver/work/freecell-solver-6.0.1/xxHash-wrapper/xxHash-0.7.2/xxh3.h:627:32: warning: implicit declaration of function 'vec_vsx_ld' is invalid in C99 [-Wimplicit-function-declaration]

U64x2 const data_vec = vec_vsx_ld(0, xinput + i);