channeld: replace struct splice with struct splicing for OpenBSD.

Since it's only for transitory splicing info, the new name makes sense.

```
cc channeld/channeld.c
In file included from channeld/channeld.c:23:
./channeld/splice.h:37:8: error: redefinition of 'splice'
struct splice {
       ^
/usr/include/sys/socket.h:140:8: note: previous definition is here
struct  splice {
        ^
```

Reported-by: @grubles
Signed-off-by: Rusty Russell <[email protected]>
Fixes: ElementsProject#6486

Author: rustyrussell