payloads/external/SeaBIOS: Bump version to 1.7.5

Change-Id: Ie4b58b739ea411035b1801348e3e73e607299846
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6900
Tested-by: build bot (Jenkins)
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>