xvisor: needs gcc >= 4.9

xvisor needs gcc >= 4.9, as previous versions trigger an ICE on
dwarf2out_frame_debug_adjust_cfa() [1].

Fixes:

 - http://autobuild.buildroot.net/results/fdb/fdb5a568fb38a9d20780cd37eecb71b60b6ca96a/

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60264



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>