debian: Fix build failure installing ovs-vswitchd.conf.db(5) manpage.

ovs-vswitchd.conf.db is distributed so it's in the source directory not
the build directory.

This fixes a Debian package build failure introduced by commit 9840bdbd
"debian: Install ovs-vswitchd.conf.db(5) manpage."  I did test that commit
but the build failure didn't show up in my environment (probably I had a
stray file left over from development).