Fix redefinition error for FD_SETSIZE on OpenBSD 4.6.

--- compat/fdsetsize.h.orig	Mon May  3 12:56:05 2010
+++ compat/fdsetsize.h	Mon May  3 12:59:49 2010
@@ -71,6 +71,7 @@

 /* Increase FD_SETSIZE if SQUID_MAXFD is bigger */
 #if CHANGE_FD_SETSIZE && SQUID_MAXFD > DEFAULT_FD_SETSIZE
+#undef FD_SETSIZE
 #define FD_SETSIZE SQUID_MAXFD
 #endif