Fix compile fail for empty enum in cipher_wrap

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/cipher_wrap.c b/library/cipher_wrap.c
index da4e739..890be3c 100644
--- a/library/cipher_wrap.c
+++ b/library/cipher_wrap.c
@@ -120,8 +120,10 @@
     MBEDTLS_CIPHER_BASE_INDEX_NULL_BASE,
 #endif
 #if defined(MBEDTLS_CIPHER_MODE_XTS) && defined(MBEDTLS_AES_C)
-    MBEDTLS_CIPHER_BASE_INDEX_XTS_AES
+    MBEDTLS_CIPHER_BASE_INDEX_XTS_AES,
 #endif
+    /* Prevent compile failure due to empty enum */
+    MBEDTLS_PREVENT_EMPTY_ENUM
 };
 
 #if defined(MBEDTLS_GCM_C)