ci: Add clippy toml to configure allowed lints

multiple-crate-versions lint in thrown for "cfg-if" and "syn" as
2 different versions of these crates are being used.
`cfg-if`: 0.1.10, 1.0.0
`syn`: 1.0.109, 2.0.52

Link to the lint:
https://rust-lang.github.io/rust-clippy/master/index.html#multiple_crate_versions

These are coming from multiple dependencies and hence cannot be easily
resolved. This patch ignores these lints.

Signed-off-by: Gowtham Suresh Kumar <[email protected]>

Author: gowthamsk-arm

.clippy.toml

@@ -0,0 +1 @@
+allowed-duplicate-crates = ["cfg-if", "syn"]