Add missing include for atoi().

Required to fix build in an internal repo where this code is
imported with Copybara.

Bug: 377642856
Test: presubmit
Change-Id: I8955eb1eb84e72d388b48df0cd583c65e093eb13
diff --git a/process.cpp b/process.cpp
index b8cabf6..09a93ff 100644
--- a/process.cpp
+++ b/process.cpp
@@ -16,6 +16,8 @@
 
 #include "android-base/process.h"
 
+#include <stdlib.h>
+
 namespace android {
 namespace base {