libqrencode: link with pthread to avoid build failure

Fixes: http://autobuild.buildroot.net/results/1ca0719dd1fe0bbc528ec99b5bc2b42e985bfdca/



[Peter: only if toolchain has threads, pass --disable-thread-safety otherwise]
Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>