Dualcpu: Add extern "C" keyword in tfm_mailbox.h

Add extern "C" keyword which was lost in tfm_mailbox.h.

Change-Id: I85480ffdf6932200a761d9dc823ac6aec1482350
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/interface/include/tfm_mailbox.h b/interface/include/tfm_mailbox.h
index 2c0ccc5..9342ed6 100644
--- a/interface/include/tfm_mailbox.h
+++ b/interface/include/tfm_mailbox.h
@@ -19,6 +19,10 @@
 #include <stddef.h>
 #include "psa/client.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*
  * The number of slots in NSPE mailbox queue and SPE mailbox queue.
  * So far only one slot is supported in either NSPE mailbox queue or
@@ -126,4 +130,8 @@
     struct ns_mailbox_slot_t queue[NUM_MAILBOX_QUEUE_SLOT];
 };
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __TFM_MAILBOX_H__ */