audio: Remove build warnings

Change-Id: I3a31c4e9c500b60e377cd54bd2e1b400c10696f6
diff --git a/hal/voice.c b/hal/voice.c
index 044dc28..a8ccdc6 100644
--- a/hal/voice.c
+++ b/hal/voice.c
@@ -18,6 +18,7 @@
 /*#define LOG_NDEBUG 0*/
 #define LOG_NDDEBUG 0
 
+#include <stdlib.h>
 #include <errno.h>
 #include <math.h>
 #include <cutils/log.h>