COMP: compile issue with gcc-5.2 (closes #273)

- Superfluous #include "FieldFunctions.H" provokes warnings/errors
  with newer gcc.

src/postProcessing/functionObjects/utilities/ddt2/ddt2.C

@@ -27,7 +27,6 @@ License
 
 #include "volFields.H"
 #include "dictionary.H"
-#include "FieldFunctions.H"
 #include "fvcDdt.H"
 #include "steadyStateDdtScheme.H"