command/file: use %zu to format a size_t

`PRIoffset` was wrong, because it expects an `offset_type`
(i.e. `uint64_t`).  This broke on 32 bit machines where `size_t` has
32 bits.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1058

NEWS

@@ -1,4 +1,6 @@
 ver 0.22.4 (not yet released)
+* protocol
+  - fix "readpicture" on 32 bit machines
 * storage
   - curl: fix several WebDAV protocol bugs
 * decoder

src/command/FileCommands.cxx

@@ -296,7 +296,7 @@ public:
 			return;
 		}
 
-		response.Format("size: %" PRIoffset "\n", buffer.size);
+		response.Format("size: %zu\n", buffer.size);
 
 		if (mime_type != nullptr)
 			response.Format("type: %s\n", mime_type);