Fix Bazel build

[kimurayu45z]

https://github.com/FirebaseExtended/protobuf-rules-gen/issues/39

I could fix the broken configurations of Bazel.

The build of any directory exclude example succeeded.

The only remaining error is

% bazel build //...                                                                 
ERROR: /home/yu/src/github.com/FirebaseExtended/protobuf-rules-gen/example/BUILD:3:30: in proto_gen rule //example:schema_genproto_rules: 
Traceback (most recent call last):
        File "/home/yu/.cache/bazel/_bazel_yu/40604a59a4d009dade8a4279b557a9f0/external/com_google_protobuf/protobuf.bzl", line 146, column 28, in _proto_gen_impl
                ctx.actions.run(
Error in run: param 'outputs' may not be empty
ERROR: Analysis of target '//example:schema_genproto_rules' failed; build aborted: Analysis of target '//example:schema_genproto_rules' failed
INFO: Elapsed time: 0.067s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded, 0 targets configured)

@var-const Could you give me some ideas?

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[kimurayu45z]

@googlebot I signed it!

[kimurayu45z]

I could fix the above error in the commit just now

% bazel build //...
INFO: Analyzed 12 targets (6 packages loaded, 214 targets configured).
INFO: Found 12 targets...
INFO: Elapsed time: 61.811s, Critical Path: 20.47s
INFO: 407 processes: 22 internal, 385 linux-sandbox.
INFO: Build completed successfully, 407 total actions

 % bazel version
Build label: 4.2.0
Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Wed Aug 18 12:48:25 2021 (1629290905)
Build timestamp: 1629290905
Build timestamp as int: 1629290905

[kimurayu45z]

The changes in this PR is included in https://github.com/FirebaseExtended/protobuf-rules-gen/pull/43 so I close this PR.