Build audio.r_submix.default with BOARD_VNDK_VERSION

Add missing header unistd.h

Bug: 37342627
Test: BOARD_VNDK_VERSION=current m -j audio.r_submix.default
Change-Id: I5baa960dcdfc3d93d7cf172b23173dfb48babf88
diff --git a/modules/audio_remote_submix/audio_hw.cpp b/modules/audio_remote_submix/audio_hw.cpp
index 6c538ce..5efcdcb 100644
--- a/modules/audio_remote_submix/audio_hw.cpp
+++ b/modules/audio_remote_submix/audio_hw.cpp
@@ -24,6 +24,7 @@
 #include <sys/param.h>
 #include <sys/time.h>
 #include <sys/limits.h>
+#include <unistd.h>
 
 #include <cutils/compiler.h>
 #include <cutils/properties.h>