Util.h should have had a forward declaration of IFields. It was a minor oversight that it was omitted, which has been corrected. The corrected version will be available in the next drop of the API, which will occur in a few weeks from now.
In the meantime, you could actually add the forward declaration to Util.h yourself, or as Lance suggested, simply include the lynch pin header file FileGDBAPI.h which includes all of the other header files.
-David Sousa