soc/amd/common/block/psp/psp_smm.c: Add missing <string.h>

'memset' needs <string.h>.

Change-Id: Idc1d72e92c97cd5139ae7439aadb575ef011129a
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42342
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/block/psp/psp_smm.c

@@ -9,6 +9,8 @@
 #include <console/console.h>
 #include <amdblocks/psp.h>
 #include <soc/iomap.h>
+#include <string.h>
+
 #include "psp_def.h"
 
 #define C2P_BUFFER_MAXSIZE 0xc00 /* Core-to-PSP buffer */