BUILD: buffer: types/{ring.h,checks.h} should include buf.h, not buffer.h

BUILD: buffer: types/{ring.h,checks.h} should include buf.h, not buffer.h

buffer.h relies on proto/activity because it contains some code and not
just type definitions. It must not be included from types files. It
should probably also be split in two if it starts to include a proto.
This causes some circular dependencies at other places.