Add missing stdlib include for sample

BUG=none
R=machenbach@chromium.org, machenbach@chromium.or
LOG=n

Review URL: https://codereview.chromium.org/1113563002

Cr-Commit-Position: refs/heads/master@{#28108}

samples/process.cc

@@ -29,6 +29,7 @@
 
 #include <include/libplatform/libplatform.h>
 
+#include <stdlib.h>
 #include <string.h>
 
 #include <map>