sparsify: Fix automake build.

Previous commit 97c57a8887e04742c58c3956385a5366c6df02c3 added a
potential bug in the sparsify build.  If virt-sparsify had used a
*.mli file then it would have tried to run a command like:

 ocamlc mlguestfs.cmxa -c foo.mli -o foo.cmi

This would fail because ocamlc doesn't know about native code
libraries (*.cmxa).  In fact virt-sparsify was not affected by this
because it doesn't use any *.mli files.

Fix this by adjusting the automake variables.