Fix implicit declaration of print_ipv6_addr

[PetitCypou]

print_ipv6_addr() is used in print_network_information() but it isn't declared beforehand, which produces warnings.

[taylor-an]

Thank you for your request. This request has been applied by adding it to w6x00_spi.h. Commit: 9a8a12649a86f0bd8b196f9b5f1b065a377742a2