gnutls: fix build failure due to non-existing gets in recent C libraries

Fixes:

  http://autobuild.buildroot.org/results/7414989c182f8fd5b3ac849b7897b8e15df0024f/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>