libfdt: Use cstr!()

This includes two changes
- Includes cstr!() to the lib.rs temporarily.
- Uses cstr!()

Bug: 277993056, Bug: 308694211
Test: TH
Change-Id: I7a8d126157cf65af117ebe779ffc4055bcb4cdda
diff --git a/libs/libfdt/tests/api_test.rs b/libs/libfdt/tests/api_test.rs
index 78bb618..78c640e 100644
--- a/libs/libfdt/tests/api_test.rs
+++ b/libs/libfdt/tests/api_test.rs
@@ -21,6 +21,7 @@
 use std::fs;
 use std::ops::Range;
 
+// TODO(b/308694211): Use cstr!() from vmbase
 macro_rules! cstr {
     ($str:literal) => {{
         CStr::from_bytes_with_nul(concat!($str, "\0").as_bytes()).unwrap()