buildroot

dependencies: check for extract tools as well

Most of the extract tools (gzip/bzip/..) we already check for explicitly
in dependencies.sh (as they are used outside GENTARGETS), but not for
xzcat.
The .xz format is used fairly rarely, and it is likely to not be available
on build hosts, so an explicit (hardcoded) check for it isn't optimal.
Instead, add the inflate tools used to DL_TOOLS_DEPENDENCIES, similar to
how we do it for svn/git/bzr/...

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>