directfb-examples: explicitly add -lstdc++ for static build

Fixes [1] and [2]:
undefined reference to `___cxa_pure_virtual'
undefined reference to `___gxx_personality_v0'

[1] http://autobuild.buildroot.net/results/bd0/bd0f4b3e41bf6330f83e3d9b58d985cc63506a3b/
[2] http://autobuild.buildroot.net/results/584/584ace383204d97fcaad37742e2e035ca78a3d10/



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>