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>

src/gallium/tests/trivial/meson.build

@@ -24,6 +24,7 @@ foreach t : ['compute', 'tri', 'quad-tex']
     '@0@.c'.format(t),
     include_directories : inc_common,
     link_with : [libmesa_util, libgallium, libpipe_loader_dynamic],
+    dependencies : dep_thread,
     install : false,
   )
 endforeach