Fix missing imports in generated code

[vearutop]

Related to swaggest/json-cli#28.

[codecov]

Codecov Report

Merging #38 (35c4bb3) into master (b602663) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #38      +/-   ##
============================================
+ Coverage     83.51%   83.56%   +0.04%     
  Complexity      859      859              
============================================
  Files            45       45              
  Lines          2099     2105       +6     
============================================
+ Hits           1753     1759       +6     
  Misses          346      346              

Impacted Files	Coverage Δ	Complexity Δ
src/JsonSchema/MarshalEnum.php	91.52% <100.00%> (+0.61%)	17.00 <0.00> (ø)
src/JsonSchema/MarshalJson.php	100.00% <100.00%> (ø)	94.00 <0.00> (ø)

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