package/systemd-bootchart: don't build manpages

Building the man pages requires xsltproc, which is not always available
on the build machine, so the build fails at configure:
    configure: error: *** xsltproc is required for man pages

But then, even when xsltproc is present, it can't actually build the man
pages, because it can't "download external entities":
    I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
    warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
    compilation error: file ./man/custom-man.xsl line 27 element import

Add a patch (sent upstream) to allow not building the man pages.

Fixes:
    http://autobuild.buildroot.org/results/e2a/e2ae38775b6acff3c3a8ea233ec70dd68b017319/
    http://autobuild.buildroot.org/results/66d/66d85622db9f7ff5a4e8d6eed3fd3b32959685cf/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>