lib/ext2fs: fix Makefile to avoid a build splat when building without VPATH

When building in the source tree, the order of the includes caused the
compiling of debugfs/journal.c while in the lib/ext2fs directory to
find the version in lib/ext2fs instead of the desired version in
e2fsck/jfs_user.h.

We need to eventually get rid of this whole mess and have only one
jfs_user.h and build the journal-related functions once in an internal
library which is used only by e2fsprogs progams.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: "Darrick J. Wong" <darrick.wong@oracle.com>