Teach coq_dune about the empty .vos produced by coqc
Teach coq_dune about the empty .vos produced by coqc

Without this the next dune command after build a vo will wipe out the
vos, breaking interactive users. Also this means dune installs the
.vos files.

ejgallego/coq,ejgallego/coq,Matafou/coq,SkySkimmer/coq,JasonGross/coq,gares/coq,silene/coq,JasonGross/coq,JasonGross/coq,Zimmi48/coq,silene/coq,Matafou/coq,gares/coq,SkySkimmer/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,ejgallego/coq,gares/coq,SkySkimmer/coq,coq/coq,coq/coq,coq/coq,gares/coq,gares/coq,Matafou/coq,silene/coq,Zimmi48/coq,coq/coq,JasonGross/coq