renamed outputBuffer.[ch] to pipe.[ch]

No CamelCase in the file name.  The output_buffer struct is going to
be renamed to music_pipe.  There are so many buffer levels in MPD, and
calling this one "output buffer" is wrong, because it's not the last
buffer before the music reaches the output devices.

src/decoder_thread.c

@@ -21,7 +21,7 @@
 #include "decoder_control.h"
 #include "decoder_internal.h"
 #include "player_control.h"
-#include "outputBuffer.h"
+#include "pipe.h"
 #include "song.h"
 #include "mapper.h"
 #include "path.h"