Use -Werror in build/make

* Remove unused local variables.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Iced5bb903902f49d7cd5cf5f572906a512c75e18
diff --git a/tools/acp/Android.bp b/tools/acp/Android.bp
index faf2034..64f5a10 100644
--- a/tools/acp/Android.bp
+++ b/tools/acp/Android.bp
@@ -5,6 +5,7 @@
 cc_binary_host {
 
     srcs: ["acp.c"],
+    cflags: ["-Wall", "-Werror"],
 
     static_libs: ["libhost"],
     name: "acp",