znc: needs gcc >= 4.7

Fixes:
http://autobuild.buildroot.net/results/aba/aba13674fb3624e0bd6a3b1e17f28883e298c8ca/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>