llvmPackages_15.openmp: add a missing darwin check dependency

the tests still don't all pass so `doCheck` is still disabled

pkgs/development/compilers/llvm/15/openmp/default.nix

@@ -11,6 +11,7 @@
 , clang-unwrapped
 , perl
 , pkg-config
+, xcbuild
 , version
 }:
 
@@ -39,6 +40,8 @@ stdenv.mkDerivation rec {
     (if stdenv.buildPlatform == stdenv.hostPlatform then llvm else targetLlvm)
   ];
 
+  nativeCheckInputs = lib.optional stdenv.hostPlatform.isDarwin xcbuild.xcrun;
+
   # Unsup:Pass:XFail:Fail
   # 26:267:16:8
   doCheck = false;