dbus-cpp: fix build with gcc 7.x

This commit adds a patch to dbus-cpp to make it build with gcc 7.x.

Fixes:

  http://autobuild.buildroot.net/results/07a7559c0efeeda16c239e0fa06259d4cd48c71b/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>