libsp: Add missing extern "C" linkage-specification

extern "C" was added to ffa_internal_api.h and sp_api.h of libsp in
order to make them able to be used from C++.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I761e468a8091df1babf5b826d3c4ae2f246e06e8
diff --git a/components/messaging/ffa/libsp/include/sp_api.h b/components/messaging/ffa/libsp/include/sp_api.h
index e188560..3c432bb 100644
--- a/components/messaging/ffa/libsp/include/sp_api.h
+++ b/components/messaging/ffa/libsp/include/sp_api.h
@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: BSD-3-Clause */
 /*
- * Copyright (c) 2020, Arm Limited and Contributors. All rights reserved.
+ * Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.
  */
 
 #ifndef LIBSP_INCLUDE_SP_API_H_
@@ -10,6 +10,10 @@
 #include "compiler.h"  // for __noreturn
 #include "ffa_api.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Interface for the SP implementation
  */
@@ -30,4 +34,8 @@
  */
 void __noreturn sp_main(struct ffa_init_info *init_info);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* LIBSP_INCLUDE_SP_API_H_ */