Add RABIT_DLL tag to definitions of rabit APIs.

[trivialfis]

• Fix CI build.
• Fix hidden symbols.

@ShvetsKS Could you try -DHIDE_CXX_SYMBOLS=ON again with this patch enabled?

Related: dmlc/xgboost#5666 (comment)

[ShvetsKS]

Thanks for fast investigation. Unfortunately I still have this undefined symbol error on 3220a69.

I fetch your changes for rabit repo, and used docker build:
sudo tests/ci_build/ci_build.sh gpu_build docker -it --build-arg CUDA_VERSION=10.0 tests/ci_build/build_via_cmake.sh -DOPEN_MP:BOOL=ON -DHIDE_CXX_SYMBOLS=ON

[trivialfis]

@ShvetsKS I will come back to this next week. It's weird as now our pip package is by default built with hidden symbols. Also I tried the docker build last week ..

[ShvetsKS]

some remark: if I use 1.1.0RC2/RC1 I can run my benchmark and there is no such error (undefined symbol: RabitGetRank). But if I try my custom docker build with -DHIDE_CXX_SYMBOLS=ON I get error.
It's a little bit confusing if 1.1.0RC2/RC1 was built with -DHIDE_CXX_SYMBOLS=ON

[trivialfis]

@ShvetsKS RC2 should be built hidden symbols as it was to fix a bug where dmlc-core from xgboost clashes with dmlc-core from tvm.

Could you try running objdump -t libxgboost.so | grep "Rabit" and see what symbols are exported?

[trivialfis]

Sorry, my bad. Rebased onto master.

[trivialfis]

It should work now.

[trivialfis]

Closes #137 .