[sleepy-discord] Fix missing header

[Adela0814]

Fixes #32359
Error message:

/root/vcpkg/buildtrees/sleepy-discord/src/129fbab21b-0fb9b07de3.clean/sleepy_discord/zlib_compression.cpp:7:3: error: ‘memset’ was not declared in this scope
    7 |   memset(&stream, 0, sizeof(stream));
      |   ^~~~~~
/root/vcpkg/buildtrees/sleepy-discord/src/129fbab21b-0fb9b07de3.clean/sleepy_discord/zlib_compression.cpp:3:1: note: ‘memset’ is defined in header ‘<cstring>’; did you forget to ‘#include <cstring>’?
    2 | #include "zlib_compression.h"
  +++ |+#include <cstring>
    3 | 

I also submit an issue to upsteam, see yourWaifu/sleepy-discord#272.

Tested all feature on the following triplets:

• x64-windows
• x86-windows
• x64-linux

Tested usage on x64-windows.

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.