Fix small CMake error when looking for the header

CMakeLists.txt

@@ -19,7 +19,7 @@ install(TARGETS ${PROJECT_NAME} EXPORT httplibConfig
     LIBRARY  DESTINATION ${CMAKE_INSTALL_LIBDIR}
     RUNTIME  DESTINATION ${CMAKE_INSTALL_BINDIR})
 
-install(DIRECTORY include/ DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/${PROJECT_NAME})
+install(FILES httplib.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/${PROJECT_NAME})
 
 install(EXPORT httplibConfig DESTINATION share/httplib/cmake)