build: IWYU support added

src/cmake/AddUnitsModule.cmake

@@ -28,7 +28,7 @@ cmake_minimum_required(VERSION 3.15)
 function(add_units_module name)
     add_library(mp-units-${name} INTERFACE)
     target_link_libraries(mp-units-${name} INTERFACE ${ARGN})
-    target_include_directories(mp-units-${name} INTERFACE
+    target_include_directories(mp-units-${name} ${units_as_system} INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
         $<INSTALL_INTERFACE:include>
     )