ebtables: fix static build

ebtables has a funky way of building the static version.
Fixes:
http://autobuild.buildroot.net/results/f5916d84194b6aa02d8eca7cbbb4e7867bb6afd4/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>