tests: Include <string> as needed

fanc999-1 · kjellahl · commit 0cc1962d7081 · 2018-09-10T15:16:53.000+02:00
Some compilers do insist that &lt;string&gt; be included so that things like
to_string() and stoi() becomes part of the std namespace.
diff --git a/tests/test_tuple_for_each.cc b/tests/test_tuple_for_each.cc
@@ -19,6 +19,7 @@
 #include <sigc++/tuple-utils/tuple_for_each.h>
 #include <iostream>
 #include <functional>
+#include <string>
 
 template <typename T_element_from>
 class for_each_simple
diff --git a/tests/test_tuple_transform_each.cc b/tests/test_tuple_transform_each.cc
@@ -19,6 +19,7 @@
 #include <sigc++/tuple-utils/tuple_transform_each.h>
 #include <utility>
 #include <functional>
+#include <string>
 
 template <typename T_element_from>
 class transform_to_string
