Forward-declare structs and use them in function pointer declarations.
Fixes build with C23.
https://bugs.gentoo.org/967014
https://github.com/davies147/astmanproxy/pull/19
--- a/src/include/astmanproxy.h
+++ b/src/include/astmanproxy.h
@@ -107,10 +107,13 @@
 	char forcebanner[80];			/* override banner output in 'standard' protocol */
 };
 
+struct mansession;
+struct message;
+
 struct iohandler {
-	int (*read) ();
-	int (*write) ();
-	int (*onconnect) ();
+	int (*read) (struct mansession *s, struct message *m);
+	int (*write) (struct mansession *s, struct message *m);
+	int (*onconnect) (struct mansession *s, struct message *m);
 	char formatname[80];
 	void *dlhandle;
 	struct iohandler *next;