Add missing include to fix compilation error of ROOT6 dictionary

Fix compilation error in compiling a ROOT6 dictionary by adding a missing #include to a classes.h file.
This will fix the compilation error in the most recent ROOT6 IBs.
Since the missing header is not specific to ROOT6, this is being requested in the main 7_1_X branch.
Since this request is purely technical, please bypass signatures if not signed in a timely manner.