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/Android.bp b/libs/nativewindow/Android.bp
index 938d426..009eec4 100644
--- a/libs/nativewindow/Android.bp
+++ b/libs/nativewindow/Android.bp
@@ -34,6 +34,12 @@
 
     clang: true,
 
+    cflags: [
+        "-Wall",
+        "-Werror",
+        "-Wno-unused-function",
+    ],
+
     cppflags: [
         "-std=c++1z"
     ],