fix logical conflict with protobuf

[alamb]

#1941 introduced a small logical conflict with #1887 which results in a compile error on master:

https://github.com/apache/arrow-datafusion/runs/5467727177?check_suite_focus=true

error: failed to run custom build command for `ballista-core v0.6.0 (/__w/arrow-datafusion/arrow-datafusion/ballista/rust/core)`

Caused by:
  process didn't exit successfully: `/github/home/target/debug/debug/build/ballista-core-8af276defa9847ff/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-env-changed=FORCE_REBUILD
  cargo:rerun-if-changed=proto/ballista.proto
  cargo:rerun-if-changed=proto/datafusion.proto

  --- stderr
  Error: "protobuf compilation failed: protoc failed: ballista.proto:402:3: \"LogicalExprNode\" is not defined.\n"
warning: build failed, waiting for other jobs to finish...
error: build failed
Error: Process completed with exit code 101.

This PR fixes the problem

[alamb]

Since this seems likely to be uncontroversial (and fixes something I broke 😢 ) merging it in