fix: define fallback GFLAGS_NS=google in config headers and remove the need of passing the -DGFLAGS_NS=google flags

[chenrui333]

What problem does this PR solve?

seeing some regression build failure in Homebrew/homebrew-core#204308

/opt/homebrew/Cellar/brpc/1.12.0/include/butil/reloadable_flags.h:63:9: error: use of undeclared identifier 'GFLAGS_NS'
   63 |     if (GFLAGS_NS::RegisterFlagValidator(flag, validate_fn)) {
      |         ^

It would be just better to define GFLAGS_NS=google in config headers

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[chenrui333]

relates to #2877

[chenBright]

In my opinion, it is better to use GFLAGS_NAMESPACE instead of GFLAGS_NS.

[chenBright]

The issue has been fixed in in #2877 .