gnutls: fix build failure related to pthreads

The pthreads autodetection poisons the linkpath thus making other
distribution libraries to take precedence over the cross ones.
Leading to failures such as:
http://autobuild.buildroot.net/results/tmp/3f979d4e2186ee31012c332fedec9591890b0b77



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