Skip cmdprivate components in GIR

Both of these were inadvertently being included in the GIR. The
OstreeCmdPrivateVTable declaration ended up causing a g-ir-compiler
error because I wanted to add a function that had a return type that was
marked as skipped (OstreeRepoAutoLock). The ostree_cmd__private__
documentation just had the wrong name.

src/libostree/ostree-cmdprivate.c

@@ -36,7 +36,7 @@ impl_ostree_generate_grub2_config (OstreeSysroot *sysroot, int bootversion, int
 }
 
 /**
- * ostree_cmdprivate: (skip)
+ * ostree_cmd__private__: (skip)
  *
  * Do not call this function; it is used to share private API between
  * the OSTree commandline and the library.

src/libostree/ostree-cmdprivate.h

@@ -29,6 +29,12 @@ G_BEGIN_DECLS
 
 gboolean _ostree_impl_system_generator (const char *ostree_cmdline, const char *normal_dir, const char *early_dir, const char *late_dir, GError **error);
 
+/**
+ * OstreeCmdPrivateVTable: (skip)
+ *
+ * Function table to share private API between the OSTree commandline and the
+ * library. Don't use this.
+ */
 typedef struct {
   gboolean (* ostree_system_generator) (const char *ostree_cmdline, const char *normal_dir, const char *early_dir, const char *late_dir, GError **error);
   gboolean (* ostree_generate_grub2_config) (OstreeSysroot *sysroot, int bootversion, int target_fd, GCancellable *cancellable, GError **error);