openbox: add missing propagated dependencies from pango

pango needs atomics support and C++ support, and those dependencies
where not propagated to the new openbox package.

Should fix:

  http://autobuild.buildroot.org/results/d3b/d3b17335f2e5720a69b4635602fef30a5ae34660/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>