[cflags] C++20 warning suppressed

We need -Wno-deprecated-anon-enum-enum-conversion to build in C++20
mode.

Test: Add --experimental_cxx_version=20 to args.gn and fx build

Change-Id: I32deea7620ee351c467852711cea994034340d0b
diff --git a/src/include/BUILD.gn b/src/include/BUILD.gn
index 2d718d6..141f492 100644
--- a/src/include/BUILD.gn
+++ b/src/include/BUILD.gn
@@ -11,8 +11,10 @@
 
     # Suppress deprecated implicit copy constructor warnings
     "-Wno-deprecated-copy",
+    "-Wno-deprecated-anon-enum-enum-conversion",
   ]
 
+
   # Once the file generation in mainline is fixed, we can remove this exception.
   # Until then, certain generated files have lower-case instead of upper-case paths.
   if (host_os == "mac") {