Core: shorten the include path of header files under secure_fw/core/include

Directly include header files under secure_fw/core/include instead
of including the whole relative paths.
It can save maintenance cost when code structure changes.

Change-Id: I3a54915b988368ca736a008056bdeb7eaa94990a
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/secure_fw/spm/spm_api.h b/secure_fw/spm/spm_api.h
index 70d9cac..3edd8a8 100644
--- a/secure_fw/spm/spm_api.h
+++ b/secure_fw/spm/spm_api.h
@@ -11,7 +11,7 @@
 /* This file contains the apis exported by the SPM to tfm core */
 #include "tfm_api.h"
 #include "spm_partition_defs.h"
-#include "secure_fw/core/include/tfm_secure_api.h"
+#include "tfm_secure_api.h"
 #include <stdbool.h>
 #ifdef TFM_PSA_API
 #include "tfm_list.h"