add missing import

src/dummy_pick_place.cpp

@@ -14,6 +14,7 @@
 
 #include <google/protobuf/text_format.h>
 #include <random>
+#include <optional>
 
 #include "connector.pb.h"
 
@@ -202,4 +203,4 @@ int main(int argc, char **argv) {
     }
 
     return 0;
-}
\ No newline at end of file
+}