Use the absolute path instead of a relative path.

The latest implementation of the path.relative function of node v6 seems
to be broken when either of the "from" or "to" paths are in a network
resource of windows, removing the leading backslashes "\\" and returning
a wrong path [0].

The new approach employs the path.resolve function that returns the
absolute path to the nan headers and seems to work fine in the case of
node v6 running in a windows box.

Tests:

* Windows 10 with node 4.3.0:
  c:\>node -e "console.log(require('path').relative('.','\\\\VBOXSRV\\vagrant'))"
  returns: \\VBOXSRV\vagrant\

* Windows 10 with node 6.5.0:
  c:\>node -e "console.log(require('path').relative('.','\\\\VBOXSRV\\vagrant'))"
  returns: VBOXSRV\vagrant\

[0] #600

include_dirs.js

@@ -1 +1 @@
-console.log(require('path').relative('.', __dirname));
+console.log(require('path').resolve(__dirname));