Disable more warnings in external/iw

Disable existing warnings and enable -Werror to prevent
any regressions.

Bug: 31434916
Test: make -j60 iw produces no warnings
Change-Id: Ia7f69acefc1eea55fbeb609e0a48e261f62c8b44
diff --git a/Android.mk b/Android.mk
index 700d516..bd5fa96 100644
--- a/Android.mk
+++ b/Android.mk
@@ -15,7 +15,9 @@
 # Silence some warnings for now. Needs to be fixed upstream. b/26105799
 LOCAL_CFLAGS += -Wno-unused-parameter \
                 -Wno-sign-compare \
-                -Wno-format
+                -Wno-format \
+                -Wno-absolute-value \
+                -Werror
 LOCAL_CLANG_CFLAGS += -Wno-enum-conversion
 
 LOCAL_LDFLAGS := -Wl,--no-gc-sections