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
2 files changed