Fix includes for `hpb` and `hpb_generator`

[tempoz]

These include paths are pointing to files that do not exist

[googleberg]

@honglooker how do you want to address unbuildable hpb bits?

[honglooker]

Hey @tempoz, thanks for taking a stab! This is likely better tackled internally after our migration is complete. I plan on tackling this; if you're curious, please feel free to ask about bug 366171162 later on -- thanks!