gnutls: make sure librt detection doesn't poison the linker flags

Just like we're passing --with-libpthread-prefix, we also need to pass
--with-librt-prefix in order to avoid having the gnutls build system
detect the librt in /usr/lib, and pass -L/usr/lib to the linker flags.

Fixes:

  http://autobuild.buildroot.org/results/fa5/fa58602cb78ffe3ae4ee389ef5cf5a37b7657c4c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>