meson: Add dep_thread dependency.
Fix this build error on Ubuntu 18.04.
/usr/bin/ld: src/util/libmesa_util.a(u_cpu_detect.c.o): undefined reference to symbol 'pthread_once@@GLIBC_2.2.5'
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110663 Suggested-by: Eric Engestrom fdo-bugs@engestrom.ch Signed-off-by: Vinson Lee vlee@freedesktop.org