samples/bpf: add missing <linux/if_vlan.h>

This fixes build error regarding redefinition:

    CLANG-bpf  samples/bpf/parse_varlen.o
  samples/bpf/parse_varlen.c:111:8: error: redefinition of 'vlan_hdr'
  struct vlan_hdr {
         ^
  ./include/linux/if_vlan.h:38:8: note: previous definition is here

So remove duplicate 'struct vlan_hdr' in sample code and include if_vlan.h
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/parse_varlen.c

@@ -6,6 +6,7 @@
  */
 #define KBUILD_MODNAME "foo"
 #include <linux/if_ether.h>
+#include <linux/if_vlan.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
 #include <linux/in.h>
@@ -108,11 +109,6 @@ static int parse_ipv6(void *data, uint64_t nh_off, void *data_end)
 	return 0;
 }
 
-struct vlan_hdr {
-	uint16_t h_vlan_TCI;
-	uint16_t h_vlan_encapsulated_proto;
-};
-
 SEC("varlen")
 int handle_ingress(struct __sk_buff *skb)
 {