pw_kvs: Restore implicitly deleted constructor
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 <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Commit-Queue: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed