package/gcc: fix gcc 4.7 build with gcc5

gcc 4.7 doesn't build with gcc5.

Patch is from DragonFlyBSD github [1]

[1] https://github.com/DragonFlyBSD/DPorts/issues/136



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>