Turn on -Wextra.

Change-Id: I3cddd6d949fdfa79312246670c37475276734f41
diff --git a/adb/Android.mk b/adb/Android.mk
index f3a4822..8d05a2e 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -14,7 +14,7 @@
 adb_version := $(shell git -C $(LOCAL_PATH) rev-parse --short=12 HEAD 2>/dev/null)-android
 
 ADB_COMMON_CFLAGS := \
-    -Wall -Werror \
+    -Wall -Wextra -Werror \
     -Wno-unused-parameter \
     -DADB_REVISION='"$(adb_version)"' \