Ignore unknown warnings.

Update the cmake flags to ignore warnings for warning flags that the
compiler doesn't know about.

Change-Id: I2a72786807ef0cc50e606dd980b6b2c9d9beb069
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116552
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

CMakeLists.txt

@@ -320,6 +320,7 @@ function(common_compile_options TARGET)
       -fno-rtti
 
       -Wno-deprecated-builtins
+      -Wno-unknown-warning-option
     )
 
     if (${DAWN_ENABLE_MSAN})