BUILD: connection: do not use VAR_ARRAY in struct tlv

It was brought by commit c44b8de99 ("CLEANUP: connection: Use `VAR_ARRAY`
in `struct tlv` definition") but breaks the build with clang. Actually it
had already been done 6 months ago by commit 4987a4744 ("CLEANUP: tree-wide:
use VAR_ARRAY instead of [0] in various definitions") then reverted by
commit 441b6c31e ("BUILD: connection: fix build on clang after the VAR_ARRAY
cleanup") which explained the same thing but didn't place a comment in the
code to justify this (in short it's just an end of struct marker).
diff --git a/include/haproxy/connection-t.h b/include/haproxy/connection-t.h
index 0c4a664..8e0ee7d 100644
--- a/include/haproxy/connection-t.h
+++ b/include/haproxy/connection-t.h
@@ -648,7 +648,7 @@
 	uint8_t type;
 	uint8_t length_hi;
 	uint8_t length_lo;
-	uint8_t value[VAR_ARRAY];
+	uint8_t value[0]; // WT: don't use VAR_ARRAY here, it's an end of struct marker
 }__attribute__((packed));
 
 struct tlv_ssl {