hw/i386: Fix broken build for non POSIX hosts

hw/i386: Fix broken build for non POSIX hosts

pc-testdev.c cannot be compiled with MinGW (and other non POSIX hosts):

  CC    i386-softmmu/hw/i386/../pc-testdev.o
qemu/hw/i386/../pc-testdev.c:38:22: warning: sys/mman.h: file not found
qemu/hw/i386/../pc-testdev.c: In function ‘test_flush_page’:
qemu/hw/i386/../pc-testdev.c:103: warning: implicit declaration of function ‘mprotect’
...

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