Use -Werror in external/libopus

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I5ca0646034c26ba70986bec9ebff5a17c678e623
diff --git a/Android.bp b/Android.bp
index 257209c..95d8c81 100644
--- a/Android.bp
+++ b/Android.bp
@@ -165,6 +165,8 @@
         "-DHAVE_LRINTF",
         "-O2",
         "-fno-math-errno",
+        "-Wall",
+        "-Werror",
     ],
     cppflags: [
         "-DBSD=1",