Use -Werror in frameworks/native/libs/nativewindow

* Suppress unused function warnings.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: If1583e0ae618f96033e0e5b661df333a8e34d858
diff --git a/libs/nativewindow/tests/Android.bp b/libs/nativewindow/tests/Android.bp
index b89c35a..20071be 100644
--- a/libs/nativewindow/tests/Android.bp
+++ b/libs/nativewindow/tests/Android.bp
@@ -23,4 +23,5 @@
     srcs: [
         "AHardwareBufferTest.cpp",
         "c_compatibility.c"],
+    cflags: ["-Wall", "-Werror"],
 }