package/fbterm: fix C++11 compliance with gcc-6

package/fbterm: fix C++11 compliance with gcc-6

gcc-6 has enabled more warnings by default, and turned some existing
warnings into errors.

Fixes (all on arc or arm):
    http://autobuild.buildroot.org/results/8c4/8c4a6cf492bccf4e1b99897bd9658e21b89b31ab/
    http://autobuild.buildroot.org/results/875/875a9deb15d47d348bb53dafdbb7ed2f2bbd3da8/
    http://autobuild.buildroot.org/results/af4/af4935f0ad2054b7dc2443e2d126fe8e50996f66/
    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>