Include string header in taskCharmPolarisation.cxx

Added include directive for string header.

Author: Mingze129

PWGHF/D2H/Tasks/taskCharmPolarisation.cxx

@@ -61,6 +61,7 @@
 #include <cstdint>
 #include <cstdlib>
 #include <numeric>
+#include <string>
 #include <vector>
 
 using namespace o2;