It's not tied to the frame_data structure any more, so it belongs by
itself.
Clean up some #includes while we're at it; in particular, frame_data.h
doesn't use anything related to tvbuffs, so don't have it gratuitiously
include tvbuff.h.
Change-Id: Ic32922d4a3840bac47007c5d4c546b8842245e0c
Reviewed-on: https://code.wireshark.org/review/13518
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Guy Harris