package/domoticz: needs gcc >= 6

domoticz fails to build with gcc 5 since bump to version 2021.1 in
commit 33b49c4a because domoticz needs
C++14 since
https://github.com/domoticz/domoticz/commit/bdf82257dc93daa78b0179a0229539553b608f6b

Fixes:
 - http://autobuild.buildroot.org/results/f4f9caa44d1836279c3806bc990a1203bf743c0d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>