build: use correct rpc.h for lockd

On cygwin, the build failed with:

In file included from ./rpc/virnetmessage.h:24:0,
                 from ./rpc/virnetclient.h:29,
                 from locking/lock_driver_lockd.c:31:
./rpc/virnetprotocol.h:9:21: fatal error: rpc/rpc.h: No such file or directory

* src/Makefile.am (lockd_la_CFLAGS): Add XDR_CFLAGS.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/Makefile.am

@@ -1727,7 +1727,9 @@ lockd_la_SOURCES = \
 		$(LOCK_DRIVER_LOCKD_SOURCES) \
 		$(LOCK_PROTOCOL_GENERATED) \
 		$(NULL)
-lockd_la_CFLAGS = -I$(top_srcdir)/src/conf $(AM_CFLAGS)
+lockd_la_CFLAGS = -I$(top_srcdir)/src/conf \
+		$(XDR_CFLAGS) \
+		$(AM_CFLAGS)
 lockd_la_LDFLAGS = -module -avoid-version
 lockd_la_LIBADD = ../gnulib/lib/libgnu.la libvirt-net-rpc.la libvirt-net-rpc-client.la
 augeas_DATA += locking/libvirt_lockd.aug