Fix wrong install path of header in Cmake

All the examples show <httplib.h> but this was putting it in <httplib/httplib.h>
So this fixes that to be as expected, in the root of the include dir.

Ref #421

Author: sum01

CMakeLists.txt

@@ -64,9 +64,9 @@ install(TARGETS ${PROJECT_NAME} EXPORT httplibConfig
     LIBRARY  DESTINATION ${CMAKE_INSTALL_LIBDIR}
     RUNTIME  DESTINATION ${CMAKE_INSTALL_BINDIR})
 
-install(FILES httplib.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/${PROJECT_NAME})
 
 install(EXPORT httplibConfig DESTINATION share/httplib/cmake)
+install(FILES httplib.h DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 export(TARGETS ${PROJECT_NAME} FILE httplibConfig.cmake)