www/firefox: fix build on powerpc64le

/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/third_party/pffft/src/pffft.c:110:45: error: implicit declaration of function 'vec_lde' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
inline v4sf ld_ps1(const float *p) { v4sf v=vec_lde(0,p); return vec_splat(vec_perm(v, v, vec_lvsl(0, p)), 0); }

^