Explicitly pass -Wno-unused-function to the targets that need it.

third_party/yasm/BUILD.gn

Index: third_party/yasm/BUILD.gn
diff --git a/third_party/yasm/BUILD.gn b/third_party/yasm/BUILD.gn
index dc6c1166142a00266349d6a4650a2dfba63448dd..fcb779ac82b04ba7c2e85a75b4e869f746b9bddc 100644
--- a/third_party/yasm/BUILD.gn
+++ b/third_party/yasm/BUILD.gn
@@ -150,11 +150,17 @@ if (current_toolchain == host_toolchain) {
 
     config("re2c_warnings") {
       # re2c is missing CLOSEVOP from one switch.
-      if (is_posix || is_clang) {
-        cflags = [ "-Wno-switch" ]
+      if (is_clang) {
+        cflags = [
+          # re2c is missing CLOSEVOP from one switch.
+          "-Wno-switch",
+
+          # re2c contains many static functions in headers (because it's
+          # a C library predating C99.)
+          "-Wno-unused-function",
+        ]
       }
     }
-
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       ":yasm_config",