package/qt5/qt5knx: bump version to 5.15.2 (and fix download)

package/qt5/qt5knx: bump version to 5.15.2 (and fix download)

- bump version to 5.15.2
- change download url to original site

Fixes:

  - http://autobuild.buildroot.net/results/a043bb991a827880d6c743e63f070fc6a3b72a2f

  >>> qt5knx 5.15.2 Downloading[27m
  --2020-12-12 13:42:47--  https://github.com/qt/qtknx/archive/v5.15.2/qt5knx-5.15.2.tar.gz
  Resolving github.com (github.com)... 140.82.114.3
  Connecting to github.com (github.com)|140.82.114.3|:443... connected.
  HTTP request sent, awaiting response... 302 Found
  Location: https://codeload.github.com/qt/qtknx/tar.gz/v5.15.2/qt5knx-5.15.2 [following]
  --2020-12-12 13:42:47--  https://codeload.github.com/qt/qtknx/tar.gz/v5.15.2/qt5knx-5.15.2
  Resolving codeload.github.com (codeload.github.com)... 140.82.114.10
  Connecting to codeload.github.com (codeload.github.com)|140.82.114.10|:443... connected.
  HTTP request sent, awaiting response... 404 Not Found
  2020-12-12 13:42:47 ERROR 404: Not Found.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>