build: use correct rpc.h for virtlockd

Commit d671121d was incomplete; I hit another compile fail on
cygwin not finding the correct rpc/rpc.h.

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

src/Makefile.am

@@ -1759,6 +1759,7 @@ virtlockd_SOURCES = \
 virtlockd_CFLAGS = \
 		$(AM_CFLAGS) \
 		$(PIE_CFLAGS) \
+		$(XDR_CFLAGS) \
 		$(NULL)
 virtlockd_LDFLAGS = \
 		$(AM_LDFLAGS) \