Revert "libsigrok: mark libsigrokcxx as broken"

This reverts commit 41e81b7a.

Doxygen dependency issue for libsigrokcxx has been fixed.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>