Add a sanity check for inclusion of port_platform.h
diff --git a/src/core/lib/iomgr/executor.cc b/src/core/lib/iomgr/executor.cc
index d2a0509..e7f412a 100644
--- a/src/core/lib/iomgr/executor.cc
+++ b/src/core/lib/iomgr/executor.cc
@@ -16,6 +16,8 @@
  *
  */
 
+#include <grpc/support/port_platform.h>
+
 #include "src/core/lib/iomgr/executor.h"
 
 #include <string.h>