Declare variable length of struct vpx_tlv

3a5af972 enabled flexelint to complain
about copying many bytes into a struct member declared as a
single byte array (which it never was).

warning addressed:

proxy/cache_proxy_proto.c  469  Warning 669: Possible data overrun for
function
    'memcpy(void *, const void *, unsigned long)', argument 3 (size=988)
    exceeds argument 1 (size=1) [Reference: file proxy/cache_proxy_proto.c:
    lines 340, 348, 392, 401, 410, 439, 469]

bin/varnishd/proxy/cache_proxy_proto.c

@@ -52,7 +52,7 @@ struct vpx_tlv {
 	unsigned		magic;
 #define VPX_TLV_MAGIC		0xdeb9a4a5
 	unsigned		len;
-	char			tlv[1];
+	char			tlv[];
 };
 
 static inline int