xml-tooling-c: fix finding boost

https://hydra.nixos.org/build/303058121/nixlog/2/tail
It's similar to a few other packages in this staging-next cycle.

pkgs/by-name/xm/xml-tooling-c/package.nix

···
     pkg-config
   ];
 
+  configureFlags = [
+    "--with-boost=${boost.dev}"
+  ];
+
   env.NIX_CFLAGS_COMPILE = lib.optionalString (!stdenv.hostPlatform.isDarwin) "-std=c++14";
 
   enableParallelBuilding = true;