[runtests] Fix macOS breakage.

stdlib.h and unistd.h were deleted in a recent change when they
shouldn't have been. mkdtemp is included from stdlib.h and on macOS
requires unistd.h

Change-Id: I696f5a1a13758ed993217e07163b7aa3d919a7ae
diff --git a/system/utest/runtests-utils/posix-test-main.cpp b/system/utest/runtests-utils/posix-test-main.cpp
index 6d21c7a..6f43475 100644
--- a/system/utest/runtests-utils/posix-test-main.cpp
+++ b/system/utest/runtests-utils/posix-test-main.cpp
@@ -8,6 +8,8 @@
 #include "runtests-utils-test-globals.h"
 
 #include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
 
 #include <fbl/auto_call.h>
 #include <runtests-utils/posix-run-test.h>