Suppress inconsistent override warning in GN.

This came up when trying to roll the new glslang into Chromium.

ANGLE bug: 3088
diff --git a/BUILD.gn b/BUILD.gn
index ad19bc2..a657fa2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -143,6 +143,7 @@
     cflags_cc = [
       "-Wno-ignored-qualifiers",
       "-Wno-implicit-fallthrough",
+      "-Wno-inconsistent-missing-override",
       "-Wno-sign-compare",
       "-Wno-unused-variable",
     ]