xtea: fix unused variable warning for config_small

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/xtea.c

@@ -39,10 +39,12 @@ static void xtea_crypt_ecb(AVXTEA *ctx, uint8_t *dst, const uint8_t *src,
                            int decrypt, uint8_t *iv)
 {
     uint32_t v0, v1;
+#if !CONFIG_SMALL
     uint32_t k0 = ctx->key[0];
     uint32_t k1 = ctx->key[1];
     uint32_t k2 = ctx->key[2];
     uint32_t k3 = ctx->key[3];
+#endif
 
     v0 = AV_RB32(src);
     v1 = AV_RB32(src + 4);