qstringalgorithms: include <iterator> in favor of <string.h>

The switch from <string> to <string.h> dropped the definition of
std::size. Include <iterator> since <string.h> is no longer used to
pull in std::size.

Amends: dc2ae08e02.

Pick-to: 6.7
Change-Id: Ib742538eb5d21c77fcae7ee9abb6d5329bbcfd63
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>

src/corelib/text/qstringalgorithms.h

@@ -13,8 +13,7 @@
 #endif
 
 #include <algorithm>        // std::find
-
-#include <string.h>         // for memchr
+#include <iterator>         // std::size
 
 #include <QtCore/q20type_traits.h>      // q20::is_constant_evaluated