process-tools-darwin: EXIT_FAILURE comes from stdlib.h
No idea why this is failing only in our builds (and not, say, Bazel CI), but I
ran into the following when bootstrapping Bazel 3.3.0 on macOS:
```
src/main/tools/process-tools-darwin.cc:48:5: error: use of undeclared identifier 'EXIT_FAILURE'
DIE("Expected only one event from the kevent call; got %d", nev);
```
In response, let's pull in stdlib.h.
Resolves #11640.
Testing Done:
- bootstrapped Bazel 3.3.0 on macOS 10.15 (Catalina) w/ Xcode 10.
Closes #11641.
PiperOrigin-RevId: 319384680
diff --git a/src/main/tools/process-tools-darwin.cc b/src/main/tools/process-tools-darwin.cc
index 277796d..03262d3 100644
--- a/src/main/tools/process-tools-darwin.cc
+++ b/src/main/tools/process-tools-darwin.cc
@@ -16,6 +16,7 @@
#include <inttypes.h>
#include <signal.h>
#include <stdint.h>
+#include <stdlib.h>
#include <sys/event.h>
#include <sys/sysctl.h>
#include <unistd.h>
