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/test/write_prsv.cpp b/vpnor/test/write_prsv.cpp
index f880476..b89b954 100644
--- a/vpnor/test/write_prsv.cpp
+++ b/vpnor/test/write_prsv.cpp
@@ -10,7 +10,9 @@
 
 #include "common.h"
 #include "mbox.h"
+extern "C" {
 #include "flash.h"
+}
 
 #include "vpnor/test/tmpd.hpp"