graphics/vapoursynth-waifu2x-w2xc: unbreak after r562653

./Waifu2x-w2xc/Waifu2x-w2xc.cpp:71:9: error: use of undeclared identifier 'w2xconv_convert_rgb_f32'

if (w2xconv_convert_rgb_f32(d->conv,
    ^