siege: fix build with clang

The build was failing on systems using clang due to an integer conversion warning being treated as an error.
Followed a similar fix in the hombrew formula to unbreak.

Fixes: #286775

pkgs/by-name/si/siege/package.nix

@@ -33,6 +33,9 @@ stdenv.mkDerivation rec {
     "--with-zlib=${zlib.dev}"
   ];
 
+  # Borrowed solution from homebrew: https://github.com/Homebrew/homebrew-core/blob/1c7c95183c0984a84b1680422afab6578c300a27/Formula/s/siege.rb#L31
+  CFLAGS = lib.optionalString stdenv.cc.isClang "-Wno-int-conversion";
+
   meta = {
     description = "HTTP load tester";
     homepage = "https://www.joedog.org/siege-home/";