Revert "tdf#42949 Fix IWYU warnings in include/rtl/*"

This partilally reverts commit 55ca35ab,
the removal of <utility> was not correct, std::move() still requires it.

Change-Id: I1f905e332874b0aa72f537e37251f822258577d6
Reviewed-on: https://gerrit.libreoffice.org/66033
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/rtl/string.hxx

@@ -26,6 +26,7 @@
 #include <cstddef>
 #include <new>
 #include <ostream>
+#include <utility>
 #include <string.h>
 
 #if defined LIBO_INTERNAL_ONLY

include/rtl/ustring.hxx

@@ -26,6 +26,7 @@
 #include <cstddef>
 #include <new>
 #include <ostream>
+#include <utility>
 
 #if defined LIBO_INTERNAL_ONLY
 #include <string_view>