annotate examples/serialize_deserialize.rs with correct features

[alexanderkjall]

Without this patch, this fails to compile:

cargo test --no-default-features --features serde_json

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.76%. Comparing base (237f081) to head (e9eda8e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   42.75%   42.76%   +0.01%     
==========================================
  Files           4        4              
  Lines        3878     3877       -1     
==========================================
  Hits         1658     1658              
+ Misses       2220     2219       -1     

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[gz]

Thank you!