greybus: audio_manager: add missing header

There's a definition missing in audio_manager causing the kernel build
to fail:

  CC [M]  ./greybus/audio_manager.o
./greybus/audio_manager.c:22:8: warning: type defaults to 'int' in declaration of 'DEFINE_IDA' [-Wimplicit-int]
error, forbidden warning: audio_manager.c:22
./kernel/scripts/Makefile.build:308: recipe for target './greybus/audio_manager.o' failed

Including linux/idr.h fixes the issue.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>