pipe: make read-only functions "pure"

Enable gcc optimizations.

src/pipe.h

@@ -20,6 +20,7 @@
 #ifndef MPD_PIPE_H
 #define MPD_PIPE_H
 
+#include <glib.h>
 #include <stdbool.h>
 
 #ifndef NDEBUG
@@ -38,6 +39,7 @@ struct music_pipe;
 /**
  * Creates a new #music_pipe object.  It is empty.
  */
+G_GNUC_MALLOC
 struct music_pipe *
 music_pipe_new(void);
 
@@ -70,6 +72,7 @@ music_pipe_contains(const struct music_pipe *mp,
  * Returns the first #music_chunk from the pipe.  Returns NULL if the
  * pipe is empty.
  */
+G_GNUC_PURE
 const struct music_chunk *
 music_pipe_peek(const struct music_pipe *mp);
 
@@ -96,9 +99,11 @@ music_pipe_push(struct music_pipe *mp, struct music_chunk *chunk);
 /**
  * Returns the number of chunks currently in this pipe.
  */
+G_GNUC_PURE
 unsigned
 music_pipe_size(const struct music_pipe *mp);
 
+G_GNUC_PURE
 static inline bool
 music_pipe_empty(const struct music_pipe *mp)
 {