Make function only used internally static.

src/pio_dbuffer.c

@@ -124,7 +124,8 @@ size_t dbuffer_data_size ( struct dBuffer *dbuffer )
   return data_size;
 }
 
-size_t dbuffer_free ( struct dBuffer *dbuffer )
+static size_t
+dbuffer_free ( struct dBuffer *dbuffer )
 {
   size_t free_size;
 

src/pio_impl.h

@@ -48,7 +48,6 @@ int       dbuffer_push(struct dBuffer *, const void *, size_t);
 size_t    dbuffer_data_size ( struct dBuffer * );
 int       dbuffer_reset ( struct dBuffer * );
 void      dbuffer_cleanup ( struct dBuffer ** );
-size_t    dbuffer_free ( struct dBuffer * );
 
 /* pio_list_set.c */
 listSet *listSetNew(valDestroyFunction, eqPredicate);