libostree: Add new OstreeSysroot class

At the moment, just a container for a path, but we will start moving
admin functionality here.

src/libostree/ostree-types.h

@@ -27,6 +27,7 @@
 G_BEGIN_DECLS
 
 typedef struct OstreeRepo OstreeRepo;
+typedef struct OstreeSysroot OstreeSysroot;
 typedef struct OstreeMutableTree OstreeMutableTree;
 typedef struct OstreeRepoFile OstreeRepoFile;