Conditionally include an include file that's not available on Mac OS.

Change-Id: I473721c352bf16f08c141c3040efddf799e5a666
diff --git a/src/dalvik_system_Zygote.cc b/src/dalvik_system_Zygote.cc
index 49556bf..099ed27 100644
--- a/src/dalvik_system_Zygote.cc
+++ b/src/dalvik_system_Zygote.cc
@@ -18,7 +18,6 @@
 #include <paths.h>
 #include <signal.h>
 #include <stdlib.h>
-#include <sys/prctl.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -32,6 +31,10 @@
 #include "ScopedUtfChars.h"
 #include "thread.h"
 
+#if defined(HAVE_PRCTL)
+#include <sys/prctl.h>
+#endif
+
 namespace art {
 
 namespace {