vcl: fix undeclared UDP_SEGMENT for centos 8

Old distros Centos 8 / Ubuntu 18.04 header files doesn't have UDP_SEGMENT
declared, define UDP_SEGMENT to right value if not defined.

Type: fix
Fixes: eff5f7aea8 ("vcl: ldp support for ip_pktinfo")

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Change-Id: I99314b895e7d09962a36e7f5582c09d0d77563dc

src/vcl/ldp.c

@@ -63,6 +63,10 @@
 #define SOCKADDR_GET_SA(__addr) _addr;
 #endif
 
+#ifndef UDP_SEGMENT
+#define UDP_SEGMENT 103
+#endif
+
 typedef struct ldp_worker_ctx_
 {
   u8 *io_buffer;