smbios.h: add missing SKU field to type3 table

The type3 SMBIOS table has a non-optional SKU field at the end,
which causes a parsing error when missing.  Add but do not populate it.

Change-Id: I988d0626b8680740697e1db58eb6d0b87874bfde
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/17851
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/smbios.h

@@ -300,6 +300,7 @@ struct smbios_type3 {
 	u8 number_of_power_cords;
 	u8 element_count;
 	u8 element_record_length;
+	u8 sku_number;
 	char eos[2];
 } __attribute__((packed));