haskell-configuration-ghc-7.6.x.nix: add missing definition for "xhtml"

hackage-packages.nix contains no "xhtml" attribute because our default
compiler -- GHC 7.10.x -- has this package as a core module. Now, the
package is missing for older versions of GHC. This patch Remedies that
issue for 7.6.x. Older compilers probably need a similar fix.

Closes https://github.com/NixOS/nixpkgs/pull/7510.