Fix missing #includes in hardware/libhardware.

Change-Id: If7c72054bf79b89c3d7a544956da671c3b404269
diff --git a/modules/usbaudio/alsa_device_proxy.c b/modules/usbaudio/alsa_device_proxy.c
index 081c05b..676f288 100644
--- a/modules/usbaudio/alsa_device_proxy.c
+++ b/modules/usbaudio/alsa_device_proxy.c
@@ -20,6 +20,8 @@
 
 #include <log/log.h>
 
+#include <errno.h>
+
 #include "alsa_device_proxy.h"
 
 #include "logging.h"