net: add missing include file

To fix building error:

CC    net/vde.o
net/vde.c: In function ‘vde_cleanup’:
net/vde.c:65:5: error: implicit declaration of function ‘qemu_set_fd_handler’ [-Werror=implicit-function-declaration]
net/vde.c:65:5: error: nested extern declaration of ‘qemu_set_fd_handler’ [-Werror=nested-externs]
cc1: all warnings being treated as errors

Signed-off-by: Liming Wang <walimisdev@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

net/vde.c

@@ -29,6 +29,7 @@
 #include "clients.h"
 #include "qemu-common.h"
 #include "qemu/option.h"
+#include "qemu/main-loop.h"
 
 typedef struct VDEState {
     NetClientState nc;