Use ninja when building llvm-ompss2

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/llvm-ompss2/clang.nix

@@ -3,6 +3,7 @@
 , lib
 , fetchFromGitHub
 , cmake
+, ninja
 , bash
 , python3
 , perl
@@ -62,6 +63,7 @@ in stdenv.mkDerivation {
   nativeBuildInputs = [
     bash
     cmake
+    ninja
     elfutils
     llvmPackages_latest.lld
     pkg-config