[APP] Fix hexagon_benchmarks build (use two-var prefetch)

[masahi]

Fix the following error when compiling hexgaon_benchmarks app:

Halide/distrib/include/Halide.h:24829:77: error: ‘Halide::Func& Halide::Func::prefetch(const T&, Halide::VarOrRVar, int, Halide::PrefetchBoundStrategy) [with T = Halide::GeneratorInput<Halide::Buffer<unsigned char> >]’ is deprecated: Call prefetch() with the two-var form instead. [-Werror=deprecated-declarations]
24829 |         return this->template as<Class>().Method(std::forward<Args>(args)...)

[steven-johnson]

LGTM pending green

[steven-johnson]

Failure is unrelated, landing