pkg-config.pc.in: Don't concatenate paths

See: https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path

cmake/pkg-config.pc.in

 Name: ${PROJECT_NAME}
 Description: JSON for Modern C++
 Version: ${PROJECT_VERSION}
-Cflags: -I${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}
+Cflags: -I${CMAKE_INSTALL_FULL_INCLUDEDIR}