pvmfw: Use fully qualified CStr in cstr!()

Allow the macro to be called from modules that don't import CStr.

Test: TH
Change-Id: Ibfc8d49136fbf71dae075ba6794c380e78513496
diff --git a/pvmfw/src/helpers.rs b/pvmfw/src/helpers.rs
index c230784..bace8da 100644
--- a/pvmfw/src/helpers.rs
+++ b/pvmfw/src/helpers.rs
@@ -182,6 +182,6 @@
 #[macro_export]
 macro_rules! cstr {
     ($str:literal) => {{
-        CStr::from_bytes_with_nul(concat!($str, "\0").as_bytes()).unwrap()
+        core::ffi::CStr::from_bytes_with_nul(concat!($str, "\0").as_bytes()).unwrap()
     }};
 }