selftests/bpf: Fix missing space error

Add the missing space after 'dest' variable assignment.
This change will resolve the following checkpatch.pl
script error:

ERROR: spaces required around that '+=' (ctx:VxW)
Signed-off-by: Roberto Valenzuela <valenzuelarober@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230113180257.39769-1-valenzuelarober@gmail.com

tools/testing/selftests/bpf/progs/test_xdp_vlan.c

@@ -195,7 +195,7 @@ int  xdp_prognum2(struct xdp_md *ctx)
 
 	/* Moving Ethernet header, dest overlap with src, memmove handle this */
 	dest = data;
-	dest+= VLAN_HDR_SZ;
+	dest += VLAN_HDR_SZ;
 	/*
 	 * Notice: Taking over vlan_hdr->h_vlan_encapsulated_proto, by
 	 * only moving two MAC addrs (12 bytes), not overwriting last 2 bytes