Marked ArduinoJson.h as a "system header"

CHANGELOG.md

@@ -9,6 +9,7 @@ HEAD
 * Fixed warning "conversion may alter value" (issue #914)
 * Muted warning "will change in GCC 7.1" (issue #914)
 * Added a clear error message for `StaticJsonBuffer` and `DynamicJsonBuffer`
+* Marked ArduinoJson.h  as a "system header"
 
 v6.9.0 (2019-02-26)
 ------

src/ArduinoJson.hpp

@@ -4,6 +4,12 @@
 
 #pragma once
 
+#ifdef __clang__
+#pragma clang system_header
+#elif defined __GNUC__
+#pragma GCC system_header
+#endif
+
 #include "ArduinoJson/Namespace.hpp"
 
 #include "ArduinoJson/Array/ArrayRef.hpp"