fmt-conversion.h: fix pixfmt.h #include; fixes latest Libav

fmt-conversion.h: fix pixfmt.h #include; fixes latest Libav

Change fmt-conversion.h to include <libavutil/pixfmt.h> instead of
<libavutil/avutil.h> which no longer indirectly includes the former.
This fixes compilation with latest Libav. Also remove superfluous
config.h #include.

The pixfmt.h #include is needed for 'enum PixelFormat'. avutil.h
already stopped including pixfmt.h more than a year ago, but that did
not cause visible breakage at the time. AFAIK the C standard does not
allow incomplete enum types, but it seems that GCC and other relevant
compilers accept function declarations using not-yet-defined enum
types without warnings. However, Libav now renamed 'enum PixelFormat'
to 'enum AVPixelFormat', adding a '#define PixelFormat AVPixelFormat'
for backwards compatibility. We need this compatibility define because
we're still using the PixelFormat name, but it of course does not work
if pixfmt.h is not included; thus the missing include now caused a
compilation failure.