SPM: tfm_multi_core: Add missing header inclusion

Add the missed stddef header inclusion to fix build errors.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: Ia6a0d93270445b6e14850c354205ade0ca29ffdd
diff --git a/secure_fw/spm/core/tfm_multi_core.h b/secure_fw/spm/core/tfm_multi_core.h
index d9536b8..02b64ce 100644
--- a/secure_fw/spm/core/tfm_multi_core.h
+++ b/secure_fw/spm/core/tfm_multi_core.h
@@ -9,6 +9,8 @@
 #define __TFM_MULTI_CORE_H__
 
 #include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
 
 /* Follow CMSE flag definitions */
 #define MEM_CHECK_MPU_READWRITE         (1 << 0x0)