rust: Import CStr from core

Import `CStr` from core instead of trusty_std now that it lives in core.

Bug: 242229115
Test: com.android.trusty.rust.*
Change-Id: I0b6dc2ad5b6fc4660dd491d75637cb77e7a1580c
diff --git a/keys.rs b/keys.rs
index dd88c93..c3e7276 100644
--- a/keys.rs
+++ b/keys.rs
@@ -16,9 +16,9 @@
 //! Trusty implementation of RetrieveKeyMaterial.
 
 use alloc::vec::Vec;
+use core::ffi::CStr;
 use hwkey::{Hwkey, KdfVersion, OsRollbackVersion, RollbackVersionSource};
 use kmr_common::{crypto, km_err, vec_try_with_capacity, Error};
-use trusty_std::ffi::CStr;
 
 pub(crate) mod legacy;