libiio: Fix tools not linked with pthreads

Add a patch cherry-picked from the upstream libiio project.

Fixes:
http://autobuild.buildroot.net/results/595/595dfd7a17e2bba71844536c9071f7608879e319
http://autobuild.buildroot.net/results/693/69390ba1f7b1460c3217ef88125faeeeda445f20
http://autobuild.buildroot.net/results/71e/71ecc80e891a36305b3dacbd12bf8130fe4fd31f
http://autobuild.buildroot.net/results/4d1/4d1d798ac740403e814b843083b14c9a890dfd54
http://autobuild.buildroot.net/results/00f/00ffdfce0e7780a0f5a5dfadf6bfd63e51c7bbb8
http://autobuild.buildroot.net/results/863/863fa72f443c45793df4837992c5c8dc65590c48



Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>