Fix a warning

No chroot on Windows.

The warning was:
C:\projects\tcpdump\tcpdump.c(1450): warning C4189: 'chroot_dir':
local variable is initialized but not referenced

tcpdump.c

@@ -1447,7 +1447,9 @@ main(int argc, char **argv)
 	char ebuf[PCAP_ERRBUF_SIZE];
 	char VFileLine[PATH_MAX + 1];
 	char *username = NULL;
+#ifndef _WIN32
 	const char *chroot_dir = NULL;
+#endif
 	char *ret = NULL;
 	char *end;
 #ifdef HAVE_PCAP_FINDALLDEVS