Fixed nullptr_t is not declared in alpine.

[limingxinleo]

我在文件里添加了以下代码，但还是编译不过

#include <cstddef>

我再尝试一下，找一下问题出在哪里

[codecov]

Codecov Report

Merging #3488 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3488      +/-   ##
==========================================
- Coverage   42.33%   42.25%   -0.09%     
==========================================
  Files          75       75              
  Lines       14371    14371              
==========================================
- Hits         6084     6072      -12     
- Misses       8287     8299      +12     

Impacted Files	Coverage Δ
src/protocol/redis.cc	60.30% <0.00%> (ø)
src/server/worker.cc	48.26% <0.00%> (-1.74%)	⬇️
src/coroutine/socket.cc	42.15% <0.00%> (-0.56%)	⬇️
src/os/async_thread.cc	73.05% <0.00%> (-0.46%)	⬇️

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[limingxinleo]

@matyhtf 峰哥，我测了一下 nullptr_t 改成 std::nullptr_t，在 alpine 里就可以编译过了。。

我也不是很懂这块。。。