Fix the draft build.

Makefile.draft got renamed to Makefile.unix in e91dfa0f, but
 build_draft.sh was not updated.

doc/build_draft.sh

@@ -59,7 +59,7 @@ done
 cp -a "${toplevel}"/src/opus_demo.c "${destdir}"/src/
 cp -a "${toplevel}"/src/opus_compare.c "${destdir}"/src/
 cp -a "${toplevel}"/celt/opus_custom_demo.c "${destdir}"/celt/
-cp -a "${toplevel}"/Makefile.draft "${destdir}"/Makefile
+cp -a "${toplevel}"/Makefile.unix "${destdir}"/Makefile
 cp -a "${toplevel}"/opus_sources.mk "${destdir}"/
 cp -a "${toplevel}"/celt_sources.mk "${destdir}"/
 cp -a "${toplevel}"/silk_sources.mk "${destdir}"/