lib: Fix dependencies on ofp-errors.inc and ofp-msgs.inc.

lib: Fix dependencies on ofp-errors.inc and ofp-msgs.inc.

Commit e21a4c0271e6f (lib/automake.mk: Fix dependency typos.) was
crossported from master to fix some observed Debian build failures, but it
was not crossported correctly.  The version on master (commit 5b3c194faa61)
did not include the $(srcdir) prefixes because master had a previous commit
ab70cd304210e (Makefiles: Stop distributing files because building them
requires Python.) that is not on branch-2.4, and when I did the crossport I
failed to pay enough attention to realize that retaining $(srcdir) for the
target was incorrect: an .lo file would always be in the builddir.  Thus,
this commit fixes the problem.

Reported-by: Alex Wang <alexw@nicira.com>
Reported-at: http://openvswitch.org/pipermail/dev/2015-July/057779.html
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>
Tested-by: Alex Wang <alexw@nicira.com>