Set INSTALL_INTERFACE to CMAKE_INSTALL_INCLUDEDIR

[opoplawski]

While packaging msgpack-c for Fedora, I needed to install into /usr/include/msgpack-c to avoid conflicting with the msgpack C++ package. But then packages that used it would not look in the correct location for the msgpack.h file. This fixes that issue.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.35%. Comparing base (0299d94) to head (def509f).
⚠️ Report is 7 commits behind head on c_master.
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Additional details and impacted files

@@             Coverage Diff              @@
##           c_master    #1177      +/-   ##
============================================
- Coverage     55.98%   55.35%   -0.64%     
============================================
  Files            10       10              
  Lines          1152     1120      -32     
  Branches        127      127              
============================================
- Hits            645      620      -25     
+ Misses          507      500       -7     

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[redboltz]

@opoplawski Thank you for sending the PR. Merged.