[build] Supress -Wbitfield-conversion

This warning was introduced by Clang and is being triggered by the
gpt implementation, suppress it since we generally avoid modifying
third party code.

Bug: 135166
Change-Id: I61045c5bba2fa41f248b7e371800c49b6edf7744
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/vboot_reference/+/936042
Reviewed-by: Leonard Chan <leonardchan@google.com>
diff --git a/firmware/lib/BUILD.gn b/firmware/lib/BUILD.gn
index 983ced7..a20b4b9 100644
--- a/firmware/lib/BUILD.gn
+++ b/firmware/lib/BUILD.gn
@@ -54,6 +54,9 @@
   ]
 
   cflags_c = [
+    # TODO(fxbug.dev/135166): Remove -Wno-unknown-warning-option after the next Clang roll.
+    "-Wno-unknown-warning-option",
+    "-Wno-bitfield-conversion",
     "-Wno-implicit-int-conversion",
     "-Wno-sign-compare",
     "-Wno-implicit-fallthrough",