addon: use absolute paths for the node include dirs

This fixes running gyp_addon from across filesystems on Windows.

This is essentially a gyp bug where it's not relativizing properly
across filesystems. See TooTallNate/node-gyp#15 for the gory details.

tools/addon.gypi

@@ -4,9 +4,9 @@
     'product_extension': 'node',
     'product_prefix': '',
     'include_dirs': [
-      '../src',
+      '<(node_root_dir)/src',
-      '../deps/uv/include',
+      '<(node_root_dir)/deps/uv/include',
-      '../deps/v8/include'
+      '<(node_root_dir)/deps/v8/include'
     ],
 
     'conditions': [