Add xdp-synproxy to bpf-examples

this code is from kernel bpf selftests xdp synproxy, removed the
tc part for simplicity, shows an exmaple of using libxdp
to attach xdp synproxy program on network interface.

if port is not in allowed ports, the packet will be dropped
by xdp synproxy by default, this would break tcp connections
to ports that user does not want to do synproxy, change the
default to allow connection pass through.

Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>

headers/vmlinux/vmlinux_common.h

@@ -1,6 +1,13 @@
 #ifndef __VMLINUX_COMMON_H__
 #define __VMLINUX_COMMON_H__
 
+enum {
+	false = 0,
+	true = 1,
+};
+
+typedef _Bool bool;
+
 struct list_head {
 	struct list_head *next;
 	struct list_head *prev;