docs: Always build ovs-vswitchd.conf.db.5 in srcdir.

docs: Always build ovs-vswitchd.conf.db.5 in srcdir.

Otherwise we build it in the build directory by default, which breaks the
Debian package build (unless we're working from a "make dist" created
tarball since that has ovs-vswitchd.conf.db.5 in the srcdir as part of
the distribution).

Reported-by: Ansis Atteka <aatteka@nicira.com>
Tested-by: Ansis Atteka <aatteka@nicira.com>