Squashed 'libs/EXTERNAL/libargparse/' changes from 4533c7092..ec74747d0

ec74747d0 argparse_util.hpp uses std::string. Include <string>

git-subtree-dir: libs/EXTERNAL/libargparse
git-subtree-split: ec74747d0f1f9e91882c3cf9b1dfa949abefe8d5

Author: kmurray

src/argparse_util.hpp

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