ci: add --cfg for cargo doc to disable compile_error!()

Some crates, such as vhost, use compile_error!() to indicate that
mutually exclusive feature have been specified. This leads to problems
when running cargo doc --all-features, which will inevitably trigger
these conditions and then lead to documentation not building (for this
reason, the vhost documentation currently fails to build on docs.rs for
example).

Add a custom cfg that gets passed via RUSTFLAGS in the cargo doc CI step
on whose absense these compile_error!() macros can be gated, so that
they dont cause issues when building documentation in CI

A custom cfg is needed because cargo doc does not pass `--cfg doc` down
to dependencies.

See also rust-vmm/vhost#326

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

.buildkite/test_description.json

@@ -29,7 +29,7 @@
     },
     {
       "test_name": "doc",
-      "command": "RUSTDOCFLAGS=\"-D warnings\" cargo doc --workspace --all-features --no-deps"
+      "command": "RUSTDOCFLAGS=\"-D warnings\" RUSTFLAGS=\"--cfg RUTSDOC_disable_feature_compat_errors\" cargo doc --workspace --all-features --no-deps"
     },
     {
       "test_name": "unittests-gnu",