`rr_graph_uxsdcxx.h`, `rr_graph_uxsdcxx_capnp.h` and
`rr_graph_uxsdcxx_interface.h` are generated via uxsdcxx and are checked in to
avoid requiring python3 and the uxsdcxx dependencies to build VPR.

See `$VTR_DIR/libs/librrgraph/src/base/SCHEMA_GENERATOR.md` for details.
