Prototype for strdup() to avoid triggering -Wint-conversion with clang 15

Index: src/slirp/misc.c
--- src/slirp/misc.c.orig
+++ src/slirp/misc.c
@@ -7,6 +7,7 @@
 
 #define WANT_SYS_IOCTL_H
 #include <slirp.h>
+#include <string.h>
 
 u_int curtime, time_fasttimo, last_slowtimo, detach_time;
 u_int detach_wait = 600000;	/* 10 minutes */