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.