sanei_wire: Fix sanei_w_void function signature

This function needs to have the same number of arguments as any other
functions passed to sanei_w_array() via its w_element argument, i.e.
sanei_w_char and sanei_w_word, in the w_option_value() implementation
in sanei/sanei_net.c.

Fixes [-Werror=cast-function-type] compiler warning.

include/sane/sanei_wire.h

@@ -96,7 +96,7 @@ Wire;
 extern void sanei_w_init (Wire *w, void (*codec_init)(Wire *));
 extern void sanei_w_exit (Wire *w);
 extern void sanei_w_space (Wire *w, size_t howmuch);
-extern void sanei_w_void (Wire *w);
+extern void sanei_w_void (Wire *w, void *);
 extern void sanei_w_byte (Wire *w, SANE_Byte *v);
 extern void sanei_w_char (Wire *w, SANE_Char *v);
 extern void sanei_w_word (Wire *w, SANE_Word *v);

sanei/sanei_wire.c

@@ -150,7 +150,7 @@ sanei_w_space (Wire * w, size_t howmuch)
 }
 
 void
-sanei_w_void (Wire * w)
+sanei_w_void (Wire * w, void __sane_unused__ * v)
 {
   DBG (3, "sanei_w_void: wire %d (void debug output)\n", w->io.fd);
 }