bazel: Disable warning about unused system_header pragma

Change-Id: I994c65bc35618adc434b9f8d4d7ac5d0d0dcdb95
diff --git a/bazel/rules.bzl b/bazel/rules.bzl
index 7fca94d..498f34a 100644
--- a/bazel/rules.bzl
+++ b/bazel/rules.bzl
@@ -22,7 +22,9 @@
 def default_cc_args():
     return {
         "deps": PERFETTO_CONFIG.deps.build_config,
-        "copts": [],
+        "copts": [
+            "-Wno-pragma-system-header-outside-header",
+        ],
         "includes": ["include"],
         "linkopts": select({
             "@perfetto//bazel:os_linux": ["-ldl", "-lrt", "-lpthread"],