[PWGLF,PWGMM] Fix cpplint errors (#18103)

Author: vkucera

PWGMM/Mult/TableProducer/percentiles.cxx

@@ -32,6 +32,7 @@
 
 #include <cstdint>
 #include <cstdlib>
+#include <string>
 
 using namespace o2;
 using namespace o2::framework;

PWGMM/Mult/Tasks/dndeta.cxx

@@ -42,6 +42,7 @@
 
 #include <algorithm>
 #include <cstdint>
+#include <string>
 #include <vector>
 
 using namespace o2;

PWGMM/Mult/Tasks/puremc-dndeta.cxx

@@ -31,6 +31,7 @@
 #include <cstdlib>
 #include <iterator>
 #include <memory>
+#include <string>
 #include <vector>
 
 using namespace o2;