Allow Send/Sync of let_cxx_string
diff --git a/src/cxx_string.rs b/src/cxx_string.rs
index 10abbce..08a58b1 100644
--- a/src/cxx_string.rs
+++ b/src/cxx_string.rs
@@ -188,7 +188,7 @@
 pub struct StackString {
     // Static assertions in cxx.cc validate that this is large enough and
     // aligned enough.
-    space: MaybeUninit<[*const (); 8]>,
+    space: MaybeUninit<[usize; 8]>,
 }
 
 impl StackString {