gcoap: cast payload to void*

This accounts for a C-nonbreaking-Rust-breaking change in RIOT APIs.

Merges: https://gitlab.com/etonomy/riot-wrappers/-/merge_requests/10
See-Also: RIOT-OS/RIOT#17958

src/gcoap.rs

@@ -308,7 +308,7 @@ impl PacketBuffer {
             coap_opt_add_opaque(
                 self.pkt,
                 optnum,
-                data.as_ptr(),
+                data.as_ptr() as _,
                 data.len().try_into().unwrap(),
             )
         }