Fix cpplint errors

Fixes errors from the following categories:

- `readability/inheritance`
- `readability/braces`
- `readability/multiline_comment`
- `whitespace/newline`
- `whitespace/empty_if_body`
- `build/include_what_you_use`
- `build/header_guard`

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;