CMake: honor CMAKE_INSTALL_INCLUDEDIR in exported target
CMake: honor CMAKE_INSTALL_INCLUDEDIR in exported target

Remove the hardcode to the 'include' directory in the CMake
exported target.

This will let the user correctly access the headers when
building a client application with a QHotkey package that
is installed with a CMAKE_INSTALL_INCLUDEDIR different
than 'include'.

Skycoder42/QHotkey