Include <memory> for CxxVector's unique_ptr shims

Fixes #548.
diff --git a/gen/src/write.rs b/gen/src/write.rs
index 419e46c..fb1a7f3 100644
--- a/gen/src/write.rs
+++ b/gen/src/write.rs
@@ -1671,6 +1671,7 @@
     writeln!(out, "  return &s[pos];");
     writeln!(out, "}}");
 
+    out.include.memory = true;
     write_unique_ptr_common(out, UniquePtr::CxxVector(element));
 
     writeln!(out, "#endif // CXXBRIDGE1_VECTOR_{}", instance);