meson: link gallium nine with pthreads

In some cases (not building with llvm, which automatically pulls in
pthreads) nine needs to be directly linked with pthreads. Fixes building
on x86 (32 bit) without llvm.

Distro bug: https://bugs.gentoo.org/670094
Fixes: 6b4c7047d5
       ("meson: build gallium nine state_tracker")
Tested-by: Rafal Lalik <rafallalik@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/gallium/targets/d3dadapter9/meson.build

@@ -53,7 +53,7 @@ libgallium_nine = shared_library(
     libswkmsdri,
   ],
   dependencies : [
-    dep_selinux, dep_expat, dep_libdrm, dep_llvm,
+    dep_selinux, dep_expat, dep_libdrm, dep_llvm, dep_thread,
     driver_swrast, driver_r300, driver_r600, driver_radeonsi, driver_nouveau,
     driver_i915, driver_svga,
   ],