meson: Always link libgallium_dri.so against dep_thread.

Somehow on my cross build the -pthread is getting lost.  All the other
deps seem to work out fine.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Tested-by: Timothy Arceri <tarceri@itsqueeze.com>
diff --git a/src/gallium/targets/dri/meson.build b/src/gallium/targets/dri/meson.build
index 0ce088e..c591b75 100644
--- a/src/gallium/targets/dri/meson.build
+++ b/src/gallium/targets/dri/meson.build
@@ -134,6 +134,7 @@
   ],
   dependencies : [
     gallium_dri_depends, dep_selinux, dep_expat, dep_libdrm, dep_llvm,
+    dep_thread,
   ],
 )