GN build: Prescribe -Wno-header-guard for direct dependents of libssl.

net/third_party/nss/ssl/BUILD.gn

Index: net/third_party/nss/ssl/BUILD.gn
diff --git a/net/third_party/nss/ssl/BUILD.gn b/net/third_party/nss/ssl/BUILD.gn
index 7f3ad2ca834640aba399ee7ef15520ad2178cbbf..55aef9d9316ec00a80d9904c6dcc57de0060db9c 100644
--- a/net/third_party/nss/ssl/BUILD.gn
+++ b/net/third_party/nss/ssl/BUILD.gn
@@ -8,6 +8,14 @@ config("ssl_config") {
   if (is_mac || is_win) {
     defines = [ "NSS_PLATFORM_CLIENT_AUTH" ]
   }
+
+  if (is_clang) {
+    cflags = [
+      # There is a broken header guard in /usr/include/nss/secmod.h:
+      # https://bugzilla.mozilla.org/show_bug.cgi?id=884072
+      "-Wno-header-guard",
+    ]
+  }
 }
 
 component("libssl") {