Fix build CLang build error by making static helper method public.

For the record, the reason this fails on Mac and not on Linux is
the following :
http://clang-developers.42468.n3.nabble.com/Access-to-static-protected-member-from-friends-of-derived-classes-td4030444.html

The Q_DECLARE_PRIVATE macro declares the qXnatAPIPrivate class and makes
it a friend of qXnatAPI, which is sufficient for G++ to give it access to
qRestAPI's static protected methods, but the Clang developers have a
different understanding of the standard, so it doesn't work on Clang.

Thanks to Steve Pieper, Florian Vichot and Sascha Zelzer for their
help fixing the issue.

See #334
Remove definition of method "qVariantMapToString" not implemented or used.

This method has originally been added by commit 7d2755

The method "qVariantMapListToString" (added in a074efc) already fulfill
the role intended by "qVariantMapToString"
Merge branch '334-clang-build-error'

* 334-clang-build-error:
  Remove definition of method "qVariantMapToString" not implemented or used.
  Fix build CLang build error by making static helper method public.