api_client: Add license info to api_client crate

In kata-containers we use the api_client crate, but it's currently
failing our cargo deny check due to missing license, and there aren't
any license files within the crate, so I haven't found a good way to
work around this.

Alternatively I'd be happy to add the license to the workspace crate
and then reference it here, but that seems to clash with the direction
of the project in #7525.

Signed-off-by: stevenhorsman <steven@uk.ibm.com>

api_client/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 authors = ["The Cloud Hypervisor Authors"]
 edition.workspace = true
+license = "Apache-2.0"
 name = "api_client"
 version = "0.1.0"