Upgrade the `fortanix-sgx-abi` dependency

0.6.1 removes the `compiler-builtins` dependency, part of RUST-142265.
The breaking change from 0.5 to 0.6 is for an update to the
`insecure_time` API [1].

I validated that `./x c library --target x86_64-fortanix-unknown-sgx`
completes successfully with this change.

Link: https://github.com/fortanix/rust-sgx/commit/a34e9767f37d6585c18bdbd31cddcadc56670d57 [1]