add valkey.conf test configuration to .gitignore.

[paulmchen]

As I used to place my redis.conf test configuration under the /src folder to run my local tests. Using this wonderful valkey replacement for Redis, I renamed my redis.conf to valkey.conf but kept it under the same /src folder for my own local test purpose.

This PR adds the following line to the ".gitignore" file (similiar as "/src/redis.conf" already existed in .gitignore). This is so that I can exclude this test configuration from my check-in when contributing commits later. Please consider including it. Thanks. (Signed-off-by)

+src/valkey.conf