Revert "remove -Wno-header-guard workaround for old versions of NSS"

build/linux/system.gyp

Index: build/linux/system.gyp
diff --git a/build/linux/system.gyp b/build/linux/system.gyp
index ffb3d98a619bbee6465d5bf647707ee9b39b5029..533379835999a09ce1eef725f837e715c785d8a9 100644
--- a/build/linux/system.gyp
+++ b/build/linux/system.gyp
@@ -1204,6 +1204,15 @@
                 ],
               },
             }],
+            ['use_openssl==0 and clang==1', {
+              'direct_dependent_settings': {
+                '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',
+                ],
+              },
+            }],
           ]
         }],
       ],