Add an OstreeSysrootUpgrader API

This moves some utility code from the ostree tool into the shared
library, which will make it easier to consume by external tools.

src/libostree/ostree-types.h

@@ -29,6 +29,7 @@ G_BEGIN_DECLS
 typedef struct OstreeRepo OstreeRepo;
 typedef struct OstreeSePolicy OstreeSePolicy;
 typedef struct OstreeSysroot OstreeSysroot;
+typedef struct OstreeSysrootUpgrader OstreeSysrootUpgrader;
 typedef struct OstreeMutableTree OstreeMutableTree;
 typedef struct OstreeRepoFile OstreeRepoFile;