Build: suppress warnings from 3rd party libraries

Warnings from 3rd parties are not relevant and are only polluting the warning output. Marking these libraries as SYSTEM suppresses warning.

https://gcc.gnu.org/onlinedocs/cpp/Invocation.html#index-I
https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html

src/Tools/plugins/widget/CMakeLists.txt

@@ -13,6 +13,7 @@ if (NOT IS_SUB_PROJECT)
 endif()
 
 include_directories(
+    SYSTEM
     ${Qt${FREECAD_QT_MAJOR_VERSION}Core_INCLUDE_DIRS}
     ${Qt${FREECAD_QT_MAJOR_VERSION}Widgets_INCLUDE_DIRS}
     ${Qt${FREECAD_QT_MAJOR_VERSION}Designer_INCLUDE_DIRS}