Include gps_iface.hpp in CMakeLists.txt install directive

[jholtom]

Pull Request Details

gps_iface.hpp included in CMakeLists.txt install directive.
Previously was not installed, and so a system installation of UHD did not have the headers available for use.

Testing Done

CMake still runs, and successfully installs gps_iface.hpp into the include folder.

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project. See CODING.md.
• I have updated the documentation accordingly.
• I have added tests to cover my changes, and all previous tests pass.
• I have checked all compat numbers if they need updating (FPGA compat,
  MPM compat, noc_shell, specific RFNoC block, ...)