Add <linux/userfaultfd.h>

This header is required to build QEMU 2.7.0 properly with our toolchain.

Taken from
   https://github.com/torvalds/linux/blob/master/include/uapi/linux/userfaultfd.h

Change-Id: Ic53dd7c71ae7d10650dfbe4ba05cf3b160681343
Commit: 9ff42d10c3b3e26d9555878f31b9a2e5c24efa57
1 file changed