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 <eric@@engestrom.ch>
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit 730ceedd)