Fix / suppress new unused warnings for mingw+clang

Bug: 69933068
Test: mmma system/core
Change-Id: I089166a979d3d8c5ada38a7745d507b555048499
diff --git a/base/Android.bp b/base/Android.bp
index 7ff02a0..ad0edf4 100644
--- a/base/Android.bp
+++ b/base/Android.bp
@@ -129,6 +129,7 @@
         },
         windows: {
             srcs: ["utf8_test.cpp"],
+            cflags: ["-Wno-unused-parameter"],
             enabled: true,
         },
     },