add spm proto headers to install

[erasaur]

first off, thanks for all your work on this project!

I came across a small issue trying to run through the example here -- currently, if I follow the instructions for installing from source, the protobuf headers aren't installed so the example fails (can't #include <sentencepiece.pb.h>). not sure if I'm just missing some steps but I figure I should submit the PR in case this is indeed an oversight.

to be precise, I'm using ubuntu and running these commands for installing (not using builtin proto):

% sudo apt-get install cmake build-essential pkg-config libgoogle-perftools-dev libprotobuf-dev protobuf-compiler
% git clone https://github.com/google/sentencepiece.git 
% cd sentencepiece
% mkdir build
% cd build
% cmake -DSPM_USE_BUILTIN_PROTOBUF=OFF ..
% make -j $(nproc)
% sudo make install
% sudo ldconfig -v

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[erasaur]

@googlebot I signed it!