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>

drivers/staging/greybus/audio_manager.c

@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/rwlock.h>
+#include <linux/idr.h>
 
 #include "audio_manager.h"
 #include "audio_manager_private.h"