libavcodec: Unconditionally build xiph.o

vorbis_parser.o is built unconditionally since 5e80fb7ff, and the
unconditionally built parts of it depend on xiph.o.

This fixes builds with --disable-everything.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit '52f954da7594c31ad94c9bcb54290145b59b27f5'

* commit '52f954da7594c31ad94c9bcb54290145b59b27f5':
  libavcodec: Unconditionally build xiph.o

Conflicts:
	libavcodec/Makefile

Merged-by: Michael Niedermayer <michaelni@gmx.at>