package/mjpg-streamer: disable opencv python and cpp example

python-numpy is an optional dependency since
https://github.com/jacksonliam/mjpg-streamer/commit/545c1a1d3abf72bb7a97c870c78e85dd0845f63a

This dependency will raise the following build failure if python-numpy
is found on the host:

aarch64_be-linux-gnu-g++: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/lib/python3/dist-packages/numpy/core/include'

because mjpg-streamer uses the python executable to retrieve the include
directory:

-- Found PythonLibs: /home/peko/autobuild/instance-0/output-1/host/aarch64_be-buildroot-linux-gnu/sysroot/usr/lib/libpython3.8.so (found version "3.8.5")
-- Found PythonInterp: /usr/bin/python3.5 (found version "3.5.3")
-- Found NUMPY: /usr/lib/python3/dist-packages/numpy/core/include

So disable python filter as well as cpp example while at it

Fixes:
 - http://autobuild.buildroot.org/results/e6795fa8ed5d8514b3e10ea1135afb31eab22b7e



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>