Shut up some warnings

src/libstore/nar-info-disk-cache.hh

@@ -10,6 +10,8 @@ class NarInfoDiskCache
 public:
     typedef enum { oValid, oInvalid, oUnknown } Outcome;
 
+    virtual ~NarInfoDiskCache() { }
+
     virtual void createCache(const std::string & uri, const Path & storeDir,
         bool wantMassQuery, int priority) = 0;
 

src/libstore/sqlite.hh

@@ -5,8 +5,8 @@
 
 #include "types.hh"
 
-class sqlite3;
-class sqlite3_stmt;
+struct sqlite3;
+struct sqlite3_stmt;
 
 namespace nix {