tap: Remove double include of util.h

If neither of __FreeBSD__, __FreeBSD_kernel__ and __DragonFly__ is defined,
util.h is included from tap-bsd.c.
Don't include it again if __OpenBSD__ is defined.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

net/tap-bsd.c

@@ -37,10 +37,6 @@
 #include <util.h>
 #endif
 
-#if defined(__OpenBSD__)
-#include <util.h>
-#endif
-
 int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required)
 {
     int fd;