Fix windows compile error

[estk]

in entry.rs:588:

async_std::os::windows::fs::symlink_file(src, dst).await
// (Could not find `fs` in `windows`)

[codecov-commenter]

Codecov Report

Merging #35 (5e78ba4) into master (748fee5) will decrease coverage by 12.24%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #35       +/-   ##
===========================================
- Coverage   80.95%   68.71%   -12.25%     
===========================================
  Files          11       11               
  Lines        2694     3733     +1039     
===========================================
+ Hits         2181     2565      +384     
- Misses        513     1168      +655     

Impacted Files	Coverage Δ
src/lib.rs	35.49% <0.00%> (-64.51%)	:arrow_down:
src/archive.rs	71.07% <0.00%> (-6.58%)	:arrow_down:
src/error.rs	35.29% <0.00%> (-4.71%)	:arrow_down:
src/entry.rs	57.85% <0.00%> (-0.09%)	:arrow_down:
tests/all.rs	99.57% <0.00%> (+<0.01%)	:arrow_up:
src/header.rs	71.45% <0.00%> (+6.74%)	:arrow_up:

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