Add string header to nchCumulantsId.cxx

Author: pravatp98

PWGCF/EbyEFluctuations/Tasks/nchCumulantsId.cxx

@@ -33,6 +33,7 @@
 #include <TPDGCode.h>
 
 #include <algorithm>
+#include <string>
 #include <vector>
 
 using namespace o2;