Rump libvirtio_ld: fix double-definition of struct cfdriver ld_cd

When libvirtio_ld builds as module, it compiles in `ld_cd` twice.
Prevent CFDRIVER_DECL from redefining it in ld_at_virtio.c when
including ioconf.c, and keep the definition in sys/dev.ld.c.

sys/rump/dev/lib/libvirtio_ld/ld_at_virtio.c

@@ -38,7 +38,15 @@ __KERNEL_RCSID(0, "$NetBSD: ld_at_virtio.c,v 1.4 2017/05/10 06:22:15 sevan Exp $
 #include <rump-sys/kern.h>
 #include <rump-sys/vfs.h>
 
+#ifdef _MODULE
+/*
+ * XXX Don't allow ioconf.c to redefine the "struct cfdriver ld_cd"
+ * XXX it will be defined in the common-code module
+ */
+#undef  CFDRIVER_DECL
+#define CFDRIVER_DECL(name, class, attr)
 #include "ioconf.c"
+#endif
 
 RUMP_COMPONENT(RUMP_COMPONENT_DEV)
 {