qtmochelpers.h: include what you need

A recent change fixed headerscheck complaining about missing std::min.
But <limits> was also missing, so included that, too. Added a comment
on <algorithm>, because I, too, thought std::min was in <utility>,
instead.

Change-Id: Ib22e78349c79673f7013a2ed26cd64b06e159ed0
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qtmochelpers.h

@@ -17,7 +17,8 @@
 
 #include <QtCore/qglobal.h>
 
-#include <algorithm>
+#include <algorithm> // std::min
+#include <limits>
 
 #if 0
 #pragma qt_no_master_include