libostree: Split off SELinux OstreeSePolicy class

It's better if this is independent from the OstreeSysroot; for
example, a policy is active in a given deployment root at once, not
for a sysroot globally.

We can also collect SELinux-related API in one place.

Unfortunately at the moment there can be only one instance of this
class per process.

src/libostree/ostree-types.h

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