Add a sanity check for inclusion of port_platform.h
diff --git a/src/core/lib/iomgr/ev_epollsig_linux.cc b/src/core/lib/iomgr/ev_epollsig_linux.cc
index 5c99c72..1e30f66 100644
--- a/src/core/lib/iomgr/ev_epollsig_linux.cc
+++ b/src/core/lib/iomgr/ev_epollsig_linux.cc
@@ -16,6 +16,8 @@
  *
  */
 
+#include <grpc/support/port_platform.h>
+
 #include "src/core/lib/iomgr/port.h"
 
 #include <grpc/grpc_posix.h>