package/paho-mqtt-cpp: bump version to 1.2.0

package/paho-mqtt-cpp: bump version to 1.2.0

"This release brings in some missing MQTT v5 features, support for
websocket headers and proxies, ALPN protocol lists, adds the builder
pattern for options, and fixes a number of bugs in both the C++ library
and the underlying C lib."

https://github.com/eclipse/paho.mqtt.cpp/releases/tag/v1.2.0

This also makes the patch obsolete, since these changes have been merged upstream.

Signed-off-by: Daniel Lang <d.lang@abatec.at>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>