codedFunctionObjectTemplate: Added #include "volFields.H"

The codedFunctionObjectTemplate is based on regionFunctionObject requiring
fvMesh.H and most manipulate volFields so it makes sense for volFields.H to be
included by default.

tutorials/basic/potentialFoam/cylinder/system/controlDict

@@ -48,10 +48,15 @@ functions
 {
     error
     {
+        type coded;
+
         // Load the library containing the 'coded' functionObject
         libs            ("libutilityFunctionObjects.so");
 
-        type coded;
+        codeInclude
+        #{
+            #include "volFields.H"
+        #};
 
         codeEnd
         #{