ARROW-2721: [C++] Fix ORC and Protocol Buffers link error

[kou]

It's introduced at 408aa5a by me.
Sorry.

[codecov-io]

Codecov Report

Merging #2146 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2146      +/-   ##
==========================================
- Coverage   86.42%   86.39%   -0.03%     
==========================================
  Files         242      230      -12     
  Lines       41536    40644     -892     
==========================================
- Hits        35896    35116     -780     
+ Misses       5640     5528     -112

Impacted Files	Coverage Δ
cpp/src/arrow/util/thread-pool-test.cc	98.91% <0%> (-0.55%)	⬇️
rust/src/builder.rs
rust/src/list.rs
rust/src/error.rs
rust/src/datatypes.rs
rust/src/buffer.rs
rust/src/lib.rs
rust/src/memory.rs
rust/src/record_batch.rs
rust/src/bitmap.rs
... and 3 more

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[xhochy]

+1, LGTM

[wesm]

Thank you for fixing this! I actually ran into this linker error myself when working on the TensorFlow symbol clash issue going on in that other patch. The good news is that revamping the symbols.map in that patch may resolve the problem (making it similar to the symbols.map in PyTorch). I will look in the next 5-7 days if no one beats me to it