gnutls: fix link-time build failures

Fixes
http://autobuild.buildroot.net/results/23da17446266bdd49ea0da0c1892d66d3ac6607f/build-end.log


And may the <BEEP> rest in peace at last...

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>