build: work around cygwin header bug

build: work around cygwin header bug

A bug in Cygwin [1] and poor error messages from gcc [2] lead
to this confusing compilation error:

qemu/qemu_monitor.c:418:9: error: passing argument 2 of 'sendmsg' from incmpatible pointer type
/usr/include/sys/socket.h:42:11: note: expected 'const struct msghdr *' but argument is of type 'struct msghdr *'

[1] http://cygwin.com/ml/cygwin/2013-05/msg00451.html
[2] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57475

* src/qemu/qemu_monitor.c (includes): Include <sys/socket.h>
before <sys/un.h>.

Signed-off-by: Eric Blake <eblake@redhat.com>