Use -Werror in build/make

* Remove unused local variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Iced5bb903902f49d7cd5cf5f572906a512c75e18
diff --git a/tools/atree/Android.bp b/tools/atree/Android.bp
index 3c3a8ef..5fbe042 100644
--- a/tools/atree/Android.bp
+++ b/tools/atree/Android.bp
@@ -9,5 +9,6 @@
         "files.cpp",
         "fs.cpp",
     ],
+    cflags: ["-Wall", "-Werror"],
     static_libs: ["libhost"],
 }