refactor: use <cstdlib> over stdlib.h

We currently use both. Consolidate on the former.

src/ipc/interfaces.cpp

@@ -13,10 +13,10 @@
 #include <util/system.h>
 
 #include <cstdio>
+#include <cstdlib>
 #include <functional>
 #include <memory>
 #include <stdexcept>
-#include <stdlib.h>
 #include <string.h>
 #include <string>
 #include <unistd.h>

src/ipc/process.cpp

@@ -10,10 +10,10 @@
 #include <util/strencodings.h>
 
 #include <cstdint>
+#include <cstdlib>
 #include <exception>
 #include <iostream>
 #include <stdexcept>
-#include <stdlib.h>
 #include <string.h>
 #include <system_error>
 #include <unistd.h>