libsigrok: add host-doxygen dependency for C++ bindings

Doxygen is required to generate xml files required by libsigrokcxx
build system.

[Thomas: add host-python (or host-python3) to the dependencies when
building the C++ bindings for libsigrok, since a Python interpreter >=
2.7 is needed at build time.]

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