alpha_encoder: fix missing include

Change-Id: I20c7d7160a81e3f2245b37925547ea2b294bc88e
diff --git a/alpha_encoder/alpha_encoder.cc b/alpha_encoder/alpha_encoder.cc
index 4402ab1..74feddb 100644
--- a/alpha_encoder/alpha_encoder.cc
+++ b/alpha_encoder/alpha_encoder.cc
@@ -17,6 +17,7 @@
  */
 
 #include <cstdio>
+#include <cstdlib>
 #include <cstring>
 #include <sstream>
 #include <string>