[Libguestfs] Fix to src/proto.c in libguestfs so it will compile with portablexdr under mingw

[Richard W.M. Jones]

Thanks, I have applied this patch.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
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