Unicodetools: compile

Add an #include for a header that was only accidentally included
transitively.

Pick-to: 5.15 6.0 6.1
Task-number: QTBUG-92822
Change-Id: Ie29bb0e065f2db712e9cf9539b15124ff0ced349
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Andreas Buhr <andreas.buhr@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/corelib/text/qunicodetools.cpp

@@ -43,6 +43,8 @@
 #include "qvarlengtharray.h"
 #include "qlibrary.h"
 
+#include <limits.h>
+
 #define FLAG(x) (1 << (x))
 
 QT_BEGIN_NAMESPACE