debian: Fix bug from commit 211b05b5 "debian: Modernize use of dh_install."

Commit 211b05b5 "debian: Modernize use of dh_install" caused build failures
in clean environments because debian/ovsdbmonitor.install now installed
files that were never created in such an environment.  It didn't cause a
build failure in my own testing because I still had old files around.
This fixes the problem.

It seems to be necessary to run "make install" once for each of binary-arch
and binary-indep, because dh_prep always deletes debian/tmp.  It is
logically not necessary but I don't see a clean and robust way to avoid it.