Fix the compile issue on Xcode 12

[dreampiggy]

This close #7

[codecov]

Codecov Report

Merging #8 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   58.92%   58.92%           
=======================================
  Files           7        7           
  Lines         577      577           
=======================================
  Hits          340      340           
  Misses        237      237           

Flag	Coverage Δ
#iOS	58.92% <100.00%> (ø)

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Impacted Files	Coverage Δ
...lasses/YYCache/YYCacheBridge/YYCache+SDAdditions.m	48.59% <100.00%> (ø)

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