Use -Werror in external/kmod

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I53ab564291f6e295c04225734427379233fde4e7
diff --git a/Android.bp b/Android.bp
index 44cd399..0736aab 100644
--- a/Android.bp
+++ b/Android.bp
@@ -35,6 +35,7 @@
         "-ffunction-sections",
         "-fdata-sections",
         "-Wall",
+        "-Werror",
         "-Wno-unused-parameter",
         "-Wno-unused-variable",
         "-Dsecure_getenv=getenv",