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/libpayload.h

@@ -45,6 +45,7 @@
 
 #include <libpayload-config.h>
 #include <ctype.h>
+#include <ipchksum.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdarg.h>
@@ -262,13 +263,6 @@ void console_add_input_driver(struct console_input_driver *in);
 #define havechar havekey
 /** @} */
 
-/**
- * @defgroup ipchecksum IP checksum functions
- * @{
- */
-unsigned short ipchksum(const void *ptr, unsigned long nbytes);
-/** @} */
-
 
 /**
  * @defgroup exec Execution functions