build: perfect build tag for mmap_unix.go

[AsterDY]

fix #564

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c71eb0) 78.57% compared to head (5cd8922) 78.57%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #567   +/-   ##
=======================================
  Coverage   78.57%   78.57%           
=======================================
  Files          69       69           
  Lines       10714    10714           
=======================================
  Hits         8418     8418           
  Misses       1930     1930           
  Partials      366      366           

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