QmlDesigner: Modernize the code a little bit

Change-Id: I24dffa459ad8a948fd58d83249cb07c827a6343f
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/array.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <array>
+
 namespace Utils {
 
 namespace Internal {