mbedtls: fix companion programs static build

Set LINK_WITH_PTHREAD to explicitly add pthread to the list of libraries.

Fixes:
http://autobuild.buildroot.net/results/e08/e087dfa2a6057b18209cd6adff2760026a9ca1db/
http://autobuild.buildroot.net/results/615/61565f1a88b7d6eea03a355ccf8dd3fe6585c4ab/
http://autobuild.buildroot.net/results/6b2/6b267b9cba37655e7d8e6d78821f96d9a8dd7d2e/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>