Move the definition of ipchksum into its own header file.

Change-Id: Ifb7c18f9ca566bd50ca138ffd8af951375089537
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/1722
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)

payloads/libpayload/include/coreboot_tables.h

@@ -31,6 +31,7 @@
 #define _COREBOOT_TABLES_H
 
 #include <arch/types.h>
+#include <ipchksum.h>
 
 struct cbuint64 {
 	u32 lo;