vold: add -Werror=format

Stop format string bugs from creeping into root level processes.

Change-Id: Ifa19ac0cacba76392040ab16cebe557b5b016706
diff --git a/Android.mk b/Android.mk
index bba6122..54abb23 100644
--- a/Android.mk
+++ b/Android.mk
@@ -53,7 +53,7 @@
 
 LOCAL_C_INCLUDES := $(common_c_includes)
 
-LOCAL_CFLAGS := 
+LOCAL_CFLAGS := -Werror=format
 
 LOCAL_SHARED_LIBRARIES := $(common_shared_libraries)