Move waitpid(2) includes to art::Exec implementation

(cherry picked from commit 446a13ff04dae7ef9c90584e7cf8e834ee841681)

Change-Id: Id5fe3dd8e6c4cef953c39b00f2a53b23da5a6247
diff --git a/runtime/utils.cc b/runtime/utils.cc
index 39a051c..68d8417 100644
--- a/runtime/utils.cc
+++ b/runtime/utils.cc
@@ -21,6 +21,8 @@
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
+#include <sys/wait.h>
+
 #include <unistd.h>
 
 #include "UniquePtr.h"