libargparse: Updating libs/EXTERNAL/libargparse/ (external git subtree from https://github.com/kmurray/libargparse.git master)

Author: kmurray

libs/EXTERNAL/libargparse/src/argparse_util.hpp

@@ -4,6 +4,7 @@
 #include <vector>
 #include <map>
 #include <memory>
+#include <string>
 
 namespace argparse {
     class Argument;