amdblocks/smm.h: Add header guards

Change-Id: I5d01c36fa4695ee42d18701a90d1b96bceb5045f
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64864
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/soc/amd/common/block/include/amdblocks/smm.h

@@ -1,5 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#ifndef _AMDBLOCKS_SMM_H_
+#define _AMDBLOCKS_SMM_H_
+
 #include <cpu/x86/msr.h>
 #include <types.h>
 
@@ -16,3 +19,5 @@ void handle_smi_store(void);
 void clear_tvalid(void);
 /* See SMITYPE_* for list possible of events. GEVENTS are handled with mainboard_smi_gpi. */
 void mainboard_handle_smi(int event);
+
+#endif