mixer: include library local header to find layout of structures

mixer: include library local header to find layout of structures

Inner this library, implementation of each features can find actual layout
of structures by including local header. Although, mixer feature is written
without the header and as the same way for applications. This brings some
inefficiencies such as usage of alloca() to keep memory objects for the
structures.

This commit includes the header for further improvements in mixer
feature.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>