dbus-c++: Remove obsolete requirement for static strings.

This patch switched dbus_set_error_const to dbus_set_error
http://codereview.chromium.org/1648006 in Error::set's implementation and made
the requirement obsolete.

BUG=none
TEST=build_packages

Change-Id: I6dc5bf6638b8d16ca9b9b6213d9aac9f88e9db58
Reviewed-on: http://gerrit.chromium.org/gerrit/6546
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: David Rochberg <rochberg@chromium.org>