package/paho-mqtt-c: disable tests and cpack docs

paho-mqtt-c by default enables the building of test materials and
install of CPack documentation:

PAHO_ENABLE_TESTING - "Build tests and run"
PAHO_ENABLE_CPACK - "Enable CPack"

Let's disable these to save a couple megabytes and time.  This is
in keeping with the generic settings in pkg-cmake.mk.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>