ssh-cipher: add getrandom and rand_core feature passthroughs (#535)

These activate the respective features of `cipher` and if it's enabled,
`aead`. This is mostly helpful for `ssh-key` to not have to directly
include these crates as dependencies just to activate these features.

Author: tarcieri

.github/workflows/ssh-cipher.yml

@@ -42,7 +42,7 @@ jobs:
           toolchain: ${{ matrix.rust }}
           target: ${{ matrix.target }}
       - uses: RustCrypto/actions/cargo-hack-install@master
-      - run: cargo hack build --target ${{ matrix.target }} --feature-powerset --exclude-features default,std
+      - run: cargo hack build --target ${{ matrix.target }} --feature-powerset --exclude-features default,std,getrandom
 
   test:
     runs-on: ubuntu-latest