Use -Werror in external/fdlibm

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I39cc0639876ece8942223ea1a0dbd1f545c31d50
diff --git a/Android.bp b/Android.bp
index 0e75a44..bb8b5b2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -119,6 +119,7 @@
         "-fno-strict-aliasing",
 
         // Disable warnings. We need a specific version of fdlibm and can't fix this upstream.
+        "-Werror",
         "-Wno-sign-compare",
         "-Wno-dangling-else",
         "-Wno-unknown-pragmas",