Suppress unused-const-variable warnings.

We don't fix these warnings and upstream has them suppressed,
let's not pretend we care about them.

Test: make -j
Change-Id: I29c91ceebd69b4bafd3f954906e91933d887202f
diff --git a/Android.bp b/Android.bp
index 06e8806..63d602a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -24,7 +24,7 @@
 cc_defaults {
     name: "protobuf-cflags-defaults",
     clang_cflags: [
-        "-Wno-error=unused-const-variable",
+        "-Wno-unused-const-variable",
         "-Wno-error=user-defined-warnings",
     ],
     cflags: [