buildroot

lttng-tools: fix build on musl

Add patch to add a missing <pthread.h> include to fix the build on musl.

Fixes:

  http://autobuild.buildroot.net/results/ce0b438ca4dcf7ccef95c7050305d8cd58fbb24a/

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