net: Add missing include statement (fix compiler warnings for MinGW)

These and some more compiler warnings were caused by a recent commit:

net/tap-win32.c:724: warning: no previous prototype for ‘tap_has_ufo’
net/tap-win32.c:729: warning: no previous prototype for ‘tap_has_vnet_hdr’
...

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

net/tap-win32.c

@@ -31,6 +31,7 @@
 #include "qemu-common.h"
 #include "clients.h"            /* net_init_tap */
 #include "net/net.h"
+#include "net/tap.h"            /* tap_has_ufo, ... */
 #include "sysemu/sysemu.h"
 #include "qemu/error-report.h"
 #include <stdio.h>