libsp: Add missing include to sp_api_defines.h

SP_RESULT definitions need to know the FF-A error code with the lowest
value. ffa_api_defines.h is now included in sp_api_defines.h so it
doesn't rely on having both header files included at usage.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ie0a99e9f2397264b4d96620927338e8228ef8292
1 file changed