Add missing #includes.

Change-Id: I81405d638f80fc96874ea0b21af3538229aa38a7
diff --git a/libcorkscrew/ptrace.c b/libcorkscrew/ptrace.c
index cbea8ca..6496d5e 100644
--- a/libcorkscrew/ptrace.c
+++ b/libcorkscrew/ptrace.c
@@ -21,6 +21,7 @@
 #include <corkscrew/ptrace.h>
 
 #include <errno.h>
+#include <stdlib.h>
 #include <sys/ptrace.h>
 #include <cutils/log.h>