Set default-features = false for rand_core

Apparently rand and rand_core need to be built with the "std" feature
set consistently, or there will be a compile error in rngs/jitter.rs.

src/rust/crypto/Cargo.toml

@@ -13,7 +13,7 @@ crate_type = ["rlib", "staticlib"]
 [dependencies]
 libc = "=0.2.39"
 digest = "=0.7.2"
-rand_core = "=0.2.0-pre.0"
+rand_core = { version = "=0.2.0-pre.0", default-features = false }
 
 external = { path = "../external" }
 smartlist = { path = "../smartlist" }
@@ -22,6 +22,7 @@ tor_log = { path = "../tor_log" }
 
 [dev-dependencies]
 rand = { version = "=0.5.0-pre.2", default-features = false }
+rand_core = { version = "=0.2.0-pre.0", default-features = false }
 
 [features]
 testing = ["tor_log/testing"]