basic: move compress.[ch] → src/basic/

basic: move compress.[ch] → src/basic/

The compression helpers are used both in journal code and in coredump
code, and there's a good chance we'll use them later for other stuff.

Let's hence move them into src/basic/, to make them a proper internal
API we can use from everywhere where that's desirable. (pstore might be
a candidate, for example)

No real code changes, just some moving around, build system
rearrangements, and stripping of journal-def.h inclusion.