Use -Werror in external/iputils

* Suppress constant-conversion warnings.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: If344a1515fc1e5911ebbe4f05d43f24f23775678
diff --git a/Android.mk b/Android.mk
index 4bc8f44..e0acf1a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -2,6 +2,8 @@
 
 iputils_cflags := \
   -D_GNU_SOURCE \
+  -Wall -Werror \
+  -Wno-constant-conversion \
   -Wno-missing-field-initializers \
   -Wno-sign-compare \
   -Wno-unused-parameter \