Fixed the build error when compiling with Swift static MUSL SDK on Linux.

[newamber]

Fixed the missing getentropy function declaration when compiling with Swift static MUSL SDK on Linux.

Add #include <unistd.h> because in musl:

➜ grep -rn "getentropy" ./include/
./include/unistd.h:182:int getentropy(void *, size_t);
./include/c++/v1/__config:274://      Use getentropy().

before:

➜ swift build --swift-sdk x86_64-swift-linux-musl
[1/1] Planning build
Building for debugging...
/home/newamber/project/mine/mtvmm/.build/checkouts/Citadel/Sources/CCitadelBcrypt/bcrypt-kdf.c:162:5: error: call to undeclared function 'getentropy'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  162 |     getentropy(key, keylen);
      |     ^
1 error generated.
[1/27] Write swift-version-5D5E3C4E9BE48293.txt

After:

➜ swift build --swift-sdk x86_64-swift-linux-musl
Building for debugging...
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
[7/7] Linking mtvmm
Build complete! (1.48s)

[newamber]

Hi @Joannis, I have submitted this PR and hope you can review it when you have time. Thank you!

[newamber]

Hi @Joannis , feel free to let me know if there’s any issue with this PR.

[Joannis]

Apologies, I hadn't seen the PR yet! I'll have a look

[Joannis]

Thanks for the PR.