Suppress nonportable include path warning.

Might be related to Goma and C files on Windows.

Bug: chromium:995046
Change-Id: If80168356a1e2a60abe8dfbf89aeed7298fd0a54
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1758998
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/tests/BUILD.gn

@@ -615,6 +615,9 @@ if (build_angle_deqp_tests && !is_fuchsia) {
         "-Wno-string-conversion",  # implicit conversion turns string literal into bool
         "-Wno-unused-local-typedef",
         "-Wno-unused-result",  # ignoring return value of function declared with 'nodiscard' attribute
+
+        # Suppress a spurious header warning. http://crbug.com/995046
+        "-Wno-nonportable-include-path",
       ]
     }