cmake: Use -Wno-poison-system-directories

This fixes the build on certain macOS systems.

Change-Id: Ifac6d77c81982d0f147dd8197cf68a33d5cc909e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/111480
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c7d0e02..fe16422 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -533,6 +533,7 @@
     -Wno-c++98-compat
     -Wno-c++98-compat-pedantic
     -Wno-format-pedantic
+    -Wno-poison-system-directories
     -Wno-return-std-move-in-c++11
     -Wno-unknown-warning-option
     -Wno-undefined-var-template