include multi-byte extern

to work around ocaml/ocaml#11449

Author: raphael-proust

src/unix/lwt_process_stubs.c

@@ -13,7 +13,13 @@
    caml_win32_multi_byte_to_wide_char, at least as of ocaml 5.0 */
 #define CAML_INTERNALS
 #if OCAML_VERSION < 50000
-#define caml_win32_multi_byte_to_wide_char win_multi_byte_to_wide_char
+/* see https://github.com/ocsigen/lwt/pull/967#issuecomment-2273495094
+ * TL;DR: some OCaml upstream issue means this extern is not included on the
+ * windows, it's added explicitly here instead. */
+CAMLextern int caml_win32_multi_byte_to_wide_char(const char* s,
+                                                  int slen,
+                                                  wchar_t *out,
+                                                  int outlen);
 #endif
 
 #include <caml/alloc.h>