Add a sanity check for inclusion of port_platform.h
diff --git a/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc b/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
index e38211f..7c0e33f 100644
--- a/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
+++ b/src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc
@@ -58,6 +58,8 @@
// using that endpoint. Because of various transitive includes in uv.h,
// including windows.h on Windows, uv.h must be included before other system
// headers. Therefore, sockaddr.h must always be included first.
+#include <grpc/support/port_platform.h>
+
#include "src/core/lib/iomgr/sockaddr.h"
#include <inttypes.h>