Makefile: Fixed broken tar by adding ".png" to figure

The figure "figures/kernel-overview-2-generic.png" was missing
the ".png" suffix in the TARFILES statment.  This was throwing
an error during the make process for the manual.  Probably an
error for quite some time and not noticed.  I added the suffix
to remove the make early exit error.

(From yocto-docs rev: 00ad7874f8da149f193caeab8fd89bbf6d8132d7)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/Makefile

@@ -325,7 +325,7 @@ ifeq ($(DOC),kernel-dev)
 XSLTOPTS = --xinclude
 ALLPREQ = html eclipse tarball
 TARFILES = kernel-dev.html kernel-dev-style.css \
-           figures/kernel-dev-title.png figures/kernel-overview-2-generic \
+           figures/kernel-dev-title.png figures/kernel-overview-2-generic.png \
            figures/kernel-architecture-overview.png figures/kernel-dev-flow.png \
            eclipse
 MANUALS = $(DOC)/$(DOC).html $(DOC)/eclipse