bf_usd is a bit slow to build, and most time is spend on the front end
of the compiler parsing USD's headers.
this change enables pre-compiled headers for the most expensive
headers to parse.
When building `bf_usd` single threaded with -j1 (for accurate measurements)
```
before Wall time: 0h 4min 30sec 931 ms 504299 ns
after Wall time: 0h 0min 50sec 808 ms 24399 ns
```
Pull Request: https://projects.blender.org/blender/blender/pulls/110027
98336cb8ca