aircrack-ng: fix statically linked build

Fixe the build failure visible at
http://autobuild.buildroot.net/results/2441d8b62323a00f32f2edb7cb781dda71ff7f68



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>