build: fix use of rpc.h on cygwin

build: fix use of rpc.h on cygwin

Caused by commit 012c25e8 splitting out a convenience library.

  CC       libvirtd_conf_la-libvirtd-config.lo
In file included from ../src/rpc/virnetmessage.h:24:0,
                 from ../src/rpc/virnetserverprogram.h:27,
                 from ../src/rpc/virnetserver.h:32,
                 from libvirtd-config.c:31:
../src/rpc/virnetprotocol.h:9:21: fatal error: rpc/rpc.h: No such file
or directory

* daemon/Makefile.am (libvirtd_conf_la_CFLAGS): Add XDR_CFLAGS.

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