tools/generate_wire.py: don't declare unused for variable.

Ubuntu clang 15.0.2-1 complains:

```
wire/peer_exp_wiregen.c:257:14: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
        for (size_t i = 0; *plen != 0; i++) {
                    ^
wire/peer_exp_wiregen.c:1373:14: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
        for (size_t i = 0; *plen != 0; i++) {
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tools/gen/impl_template

@@ -90,7 +90,7 @@ fromwire_${type_}_array(cursor, plen, ${fieldname}, ${f.size('*plen')});
 ${fieldname} = ${f.size('*plen')} ? tal_arr(${ctx}, ${typename}, 0) : NULL;
 	% endif
         % if f.is_implicit_len():
-	for (size_t i = 0; *plen != 0; i++) {
+	while (*plen != 0) {
 	% else:
 	for (size_t i = 0; i < ${f.size()}; i++) {
         % endif