pw_kvs: Restore implicitly deleted constructor

No-Docs-Update-Reason: Compiler update bugfix
Change-Id: I83e0b108d01040a93b26bf695acbd183c65f2462
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47540
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Armando Montanez <amontanez@google.com>
diff --git a/pw_kvs/public/pw_kvs/key.h b/pw_kvs/public/pw_kvs/key.h
index f449f05..789036e 100644
--- a/pw_kvs/public/pw_kvs/key.h
+++ b/pw_kvs/public/pw_kvs/key.h
@@ -46,6 +46,9 @@
   operator std::string_view() { return std::string_view{str_, length_}; }
 #endif  // __cplusplus >= 201703L
 
+  // Assignment
+  constexpr Key& operator=(const Key&) = default;
+
   // Traits
   constexpr size_t size() const { return length_; }
   constexpr size_t length() const { return length_; }