Add "cryptfs deleteuserkey" command to vold.

Bug: 19706593

(cherry-picked from commit eebf44563bf9c6f2795442e8d2bc886e4eb3bbbe)

Change-Id: I50dc4c39595c06bf0016d6a490130bbbc25de91b
diff --git a/Ext4Crypt.h b/Ext4Crypt.h
index c502b62..f5c2871 100644
--- a/Ext4Crypt.h
+++ b/Ext4Crypt.h
@@ -20,5 +20,6 @@
                       const char* value);
 int e4crypt_set_user_crypto_policies(const char *path);
 int e4crypt_create_new_user_dir(const char *user_handle, const char *path);
+int e4crypt_delete_user_key(const char *user_handle);
 
 __END_DECLS