Rename ostree-cmdprivate to drop out of introspection

I was looking at our `.gir` and noticed we had the cmdprivate bits
because the pattern for excluding headers is `-private.h`, which
didn't match `cmdprivate.h`.

src/switchroot/ostree-system-generator.c

@@ -27,7 +27,7 @@
 
 #include <libglnx.h>
 
-#include "ostree-cmdprivate.h"
+#include "ostree-cmd-private.h"
 #include "ostree-mount-util.h"
 
 static const char *arg_dest = "/tmp";