lttng-libust: fix version bumping to 2.1.1

* remove lttng-libust-dont-build-cxx-tests-when-not-available.patch
  (included in the 2.1.1 release)

fix http://autobuild.buildroot.net/results/1fcbb7b53536522b26097c858a7a95eeece02277/build-end.log



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>