tinyE: The first Warcraft was 1992 and the first Game of Thrones was 1996, so the dates help your theory, but nothing else does. :P
The Lich King was only introduced in the Frozen Throne exppack for WC3 though. So I'm not sure who copied what. The show might have taken some inspiration from Wrath of the Lich King, who knows.
The politics, political maneuvering and scheming of GOT between factions is definitely not found in Warcraft though. Might say GOT ripped off Crusader Kings 2 but CK2 was released in 2012.
Warcraft itself, as the developers have mentioned, is based and inspired by Warhammer. They failed to get a Warhammer license to make games back in the day and therefore they made their own story and world, giving birth to Warcraft. Ever since Warcraft have created content based from inspiration from other sources as well, such as Tolkien, Lovecraft etc.