Add lseek() function to IsoFileSource.

libisofs/fsource.c

@@ -92,6 +92,12 @@ int iso_file_source_read(IsoFileSource *src, void *buf, size_t count)
     return src->class->read(src, buf, count);
 }
 
+inline
+off_t iso_file_source_lseek(IsoFileSource *src, off_t offset, int flag)
+{
+    return src->class->lseek(src, offset, flag);
+}
+
 inline
 int iso_file_source_readdir(IsoFileSource *src, IsoFileSource **child)
 {