autopkgtest: copy debian/changelog for ELF package metadata

Benjamin Drungrequested to merge
elf-package-metadata into debian/latest

dpkg 1.22.21ubuntu6 set --package-metadata directly in LDFLAGS (see Launchpad bug #2071468) which requires reading debian/changelog. The autopkgtest fails due to having this warning:

dpkg-buildflags: warning: debian/changelog not found. Not setting ELF package metadata parameter.

So copy debian/changelog into the autopkgtest temporary directory.

LP: #2134532

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