Use -Werror in external/fio

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I6429477dc9f4f10be99465450955b31483a35987
diff --git a/Android.bp b/Android.bp
index e2daa40..7aacaf4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -31,6 +31,7 @@
         "-D_FILE_OFFSET_BITS=64",
         "-O3",
         "-Wall",
+        "-Werror",
         "-Wdeclaration-after-statement",
         "-Wwrite-strings",
         "-ffast-math",