libiconv: fix typedef of iconv_t

In 4fa7ed9388f3ca81d97cad2099a6f9fa9f8098de this was wrongly
imported. Now with gcc 15.x and above there is a compilation
failure when trying to use libiconv.
Tested with samba software package.

include/iconv.h

@@ -11,7 +11,7 @@ extern "C" {
 
 extern int _libiconv_version; /* Likewise */
 
-typedef long iconv_t;
+typedef void *iconv_t;
 
 extern iconv_t
 iconv_open(const char *tocode, const char *fromcode);