sytle: keep needed string includes

Author: vil02

lib/vf_settokenvalue.cpp

@@ -39,6 +39,7 @@
 #include <cstdint>
 #include <functional>
 #include <list>
+#include <string>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>

test/testmathlib.cpp

@@ -24,6 +24,7 @@
 
 #include <limits>
 #include <memory>
+#include <string>
 #include <utility>
 
 class TestMathLib : public TestFixture {