clamav: needs C++

clamav needs C++ since bump to version 0.101.1 and
https://github.com/Cisco-Talos/clamav-devel/commit/d39cb6581f3c854476044f069d2393fc44702c36

Fixes:
 - http://autobuild.buildroot.org/results/be14aa571309cda32a5963feed9fd7f220e87fe6



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>