avcodec/Makefile: add libutvideo.h to SKIPHEADERS

libutvideo.h is not a C header and thus fails building as a C file
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/Makefile

@@ -835,6 +835,7 @@ SKIPHEADERS                            += %_tablegen.h                  \
                                           %_tables.h                    \
                                           aac_tablegen_decl.h           \
                                           fft-internal.h                \
+                                          libutvideo.h                  \
                                           old_codec_ids.h               \
                                           tableprint.h                  \
                                           $(ARCH)/vp56_arith.h          \