Fix: add missing new-line-escape in jffsX-utils Makemodule.am

Fix: add missing new-line-escape in jffsX-utils Makemodule.am

In commit a888044525, various header files that were added to the
distribution tarball via EXTRA_DIST were instead added to the source
lists of their respective programs instead.

In the case of jffsX-utils, a missing escape for a new-line in the
source list caused the include/linux/jffs2.h header to not be packged
in the release tarball. This went undiscovered, as the system on which
the release tarball was built, had the same header installed installed
in the system include directory, so a `make distcheck` succeeded.

Fixes: a888044525
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>