Fixes this error:

In file included from ml_gtk.c:36:
ml_gtk.c: In function ‘ml_gtk_style_context_list_classes’:
wrappers.h:422:20: error: passing argument 2 of ‘Val_GList’ from incompatible pointer type [-Wincompatible-pointer-types]
  422 | #define Val_string copy_string_check
      |                    ^~~~~~~~~~~~~~~~~
      |                    |
      |                    value (*)(const char *) {aka long int (*)(const char *)}
ml_gtk.c:236:78: note: in expansion of macro ‘Val_string’
  236 | { return Val_GList(gtk_style_context_list_classes(GtkStyleContext_val(ctx)), Val_string); }
      |                                                                              ^~~~~~~~~~
In file included from ml_gtk.c:37:
ml_glib.h:30:42: note: expected ‘value_in’ {aka ‘long int (*)(void *)’} but argument is of type ‘value (*)(const char *)’ {aka ‘long int (*)(const char *)’}
   30 | CAMLexport value Val_GList (GList *list, value_in);
      |                                          ^~~~~~~~

--- lablgtk-3.1.4/src/wrappers.c.orig	2024-01-30 18:11:40.000000000 -0700
+++ lablgtk-3.1.4/src/wrappers.c	2024-02-02 10:44:31.679679889 -0700
@@ -113,7 +113,7 @@ CAMLexport value Val_pointer (void *ptr)
     return ret;
 }
 
-CAMLprim value copy_string_check (const char*str)
+CAMLprim value copy_string_check (void *str)
 {
     if (!str) ml_raise_null_pointer ();
     return copy_string ((char*) str);
--- lablgtk-3.1.4/src/wrappers.h.orig	2024-01-30 18:11:40.000000000 -0700
+++ lablgtk-3.1.4/src/wrappers.h	2024-02-02 10:44:24.351780589 -0700
@@ -107,7 +107,7 @@ CAMLprim value ml_some (value);
 value ml_cons (value, value);
 CAMLexport void ml_raise_null_pointer (void) Noreturn;
 CAMLexport value Val_pointer (void *);
-CAMLprim value copy_string_check (const char*);
+CAMLprim value copy_string_check (void *);
 value copy_string_or_null (const char *);
 value Val_option_string (const char *s);