fix: add graphql to peerDependencies

[jonaskuske]

This package depends on apollo-link, which lists graphql as a peer dependency. Because of this, graphql also is a (transitive) peer dependency of http-link-dataloader itself and needs to be listed as such.

Otherwise users see warnings, potential errors and if they're using a package manager that actually enforces package boundaries (like Yarn 2) it can't be used to begin with unless you fix the peerDependencies manually. (that's what happened to me)