Enable rand/std_rng feature

We recently upgraded the rand dependency and we use it behind code
feature gated on "rand-std". In that code we use `thread_rng` but this
is only available if the "std_rng" feature is turned on, however in
non-dev builds we do not enable this feature, we have a "rand-std"
feature that enables "rand/std", it should also enable "std_rng".

Enable "rand/std_rng" in the "rand-std" feature.

Cargo.toml

@@ -24,7 +24,7 @@ std = ["alloc", "secp256k1-sys/std"]
 # allow use of Secp256k1::new and related API that requires an allocator
  alloc = ["secp256k1-sys/alloc"]
  bitcoin-hashes-std = ["bitcoin_hashes/std"]
-rand-std = ["rand/std"]
+rand-std = ["rand/std", "rand/std_rng"]
 recovery = ["secp256k1-sys/recovery"]
 lowmemory = ["secp256k1-sys/lowmemory"]
 global-context = ["std"]