Clang16 will not allow implicit function pointer types by default.

Bug: https://bugs.gentoo.org/880941
Upstream Bug report and patch: https://savannah.nongnu.org/bugs/index.php?63356

Signed-off-by: Pascal Jäger <pascal.jaeger@leimstift.de>

--- a/x11.c
+++ b/x11.c
@@ -2259,7 +2259,7 @@ x_preparse_args(int *pargc, char ***pargv)
     if (isEmpty(xvile_class))
 	xvile_class = MY_CLASS;
 
-    XtSetErrorHandler(initial_error_handler);
+    XtSetErrorHandler((__attribute__((noreturn)) XtErrorHandler)runtime_error_handler);
     memset(cur_win, 0, sizeof(*cur_win));
     cur_win->top_widget = XtVaAppInitialize(&cur_win->app_context,
 					    xvile_class,
@@ -2269,7 +2269,7 @@ x_preparse_args(int *pargc, char ***pargv)
 					    Nval(XtNgeometry, NULL),
 					    Nval(XtNinput, TRUE),
 					    NULL);
-    XtSetErrorHandler(runtime_error_handler);
+    XtSetErrorHandler((__attribute__((noreturn)) XtErrorHandler)runtime_error_handler);
     dpy = XtDisplay(cur_win->top_widget);
 
     XtVaGetValues(cur_win->top_widget,