build: use XDR_CFLAGS for Cygwin build
Without XDR_CFLAGS, compilation on Cygwin fails with:
CC libvirt_driver_la-libvirt-stream.lo
In file included from libvirt-stream.c:26:0:
rpc/virnetprotocol.h:9:21: fatal error: rpc/rpc.h: No such file or directory
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit aa760cd522f514b05f3cc5f17a5845f15d0a011f)