include CHANGELOG in CODEC_SRCS

build/make/version.sh requires CHANGELOG to generate vpx_version.h
The file is already included when building the documentation. However,
documentation is not build if doxygen/php are not present.

This is necessary when using '--enable-install-srcs --enable-codec-srcs'
and 'make dist'

Change-Id: Icada883a056a4713d24934ea44e0f6969b68f9c2