[various] Squash a couple of clang warning wrt friend declarations.

persistent-data/data-structures/array.h

@@ -367,7 +367,7 @@ namespace persistent_data {
 			typename ValueTraits::value_type const &v_;
 		};
 
-		friend class resizer;
+		friend struct resizer;
 
 		//--------------------------------
 

thin-provisioning/cache_stream.h

@@ -32,7 +32,7 @@ namespace thin_provisioning {
 			chunk c_;
 		};
 
-		friend class chunk_wrapper;
+		friend struct chunk_wrapper;
 
 		block_address block_size_;
 		block_address nr_blocks_;