SPM: Add missing header include

Add missing include which fixes an issue with building
in C99 standard and TFM_ISOLATION_LEVEL != 1

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: Ie4d3effb1bc0e1e1c0dc53ec452f940e31e5da21
1 file changed