Avoid including <sys/param.h> on windows

Fixes #72
diff --git a/src/ip_icmp.c b/src/ip_icmp.c
index ffd2bc7..044f633 100644
--- a/src/ip_icmp.c
+++ b/src/ip_icmp.c
@@ -34,7 +34,9 @@
 #include "slirp.h"
 #include "ip_icmp.h"
 
+#ifndef _WIN32
 #include <sys/param.h>
+#endif
 
 #ifndef WITH_ICMP_ERROR_MSG
 #define WITH_ICMP_ERROR_MSG 0