flash: Drop __cplusplus guard

C++ concepts should go in C++ files.

Change-Id: I7d3b16c4d176b51fb8bcf976744312632322c2a8
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/vpnor/pnor_partition.cpp b/vpnor/pnor_partition.cpp
index 7961194..3ec4d22 100644
--- a/vpnor/pnor_partition.cpp
+++ b/vpnor/pnor_partition.cpp
@@ -1,9 +1,12 @@
 // SPDX-License-Identifier: Apache-2.0
 // Copyright (C) 2018 IBM Corp.
+extern "C" {
+#include "flash.h"
+}
+
 #include "pnor_partition.hpp"
 #include "pnor_partition_table.hpp"
 #include "config.h"
-#include "flash.h"
 #include "mboxd_pnor_partition_table.h"
 #include "xyz/openbmc_project/Common/error.hpp"
 #include <phosphor-logging/log.hpp>