hivex: Fix install target for systems without native OCaml compiler

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| ocamlfind install \
|           -ldconf ignore -destdir /build/buildd-hivex_1.2.6-1-ia64-iqcb38/hivex-1.2.6/debian/tmp/usr/lib/ocaml \
|           hivex \
|           META *.so *.a *.cma *.cmx *.cmxa *.cmi *.mli
| Installed /build/buildd-hivex_1.2.6-1-ia64-iqcb38/hivex-1.2.6/debian/tmp/usr/lib/ocaml/hivex/hivex.mli
| Installed /build/buildd-hivex_1.2.6-1-ia64-iqcb38/hivex-1.2.6/debian/tmp/usr/lib/ocaml/hivex/hivex.cmi
| ocamlfind: *.cmxa: No such file or directory
| make[4]: *** [install-data-hook] Error 2
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