Fix stltest compilation

The usage of std:ptrdiff_t in the test requires the #inclusion of
the cstddef header.

Task-number: QTBUG-24828
Change-Id: I7e1ca6ee010469708f8da1709563b5ebced972e5
Reviewed-by: Anssi Eteläniemi
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

config.tests/unix/stl/stltest.cpp

@@ -49,6 +49,7 @@ templates for common STL container classes.
 #include <vector>
 #include <algorithm>
 #include <iostream>
+#include <cstddef>
 
 // something mean to see if the compiler and C++ standard lib are good enough
 template<class K, class T>