Check `mbstring.func_overload` only if the mb module is installed.
Check `mbstring.func_overload` only if the mb module is installed.

Fixes https://github.com/owncloud/core/issues/14670

jbicha/server,nextcloud/server,pixelipo/server,Ardinis/server,michaelletzgus/nextcloud-server,jbicha/server,IljaN/core,sharidas/core,Ardinis/server,michaelletzgus/nextcloud-server,lrytz/core,jbicha/server,sharidas/core,bluelml/core,michaelletzgus/nextcloud-server,lrytz/core,pixelipo/server,bluelml/core,lrytz/core,pmattern/server,whitekiba/server,pollopolea/core,nextcloud/server,pmattern/server,andreas-p/nextcloud-server,pollopolea/core,owncloud/core,Ardinis/server,IljaN/core,owncloud/core,bluelml/core,cernbox/core,pollopolea/core,xx621998xx/server,andreas-p/nextcloud-server,bluelml/core,whitekiba/server,whitekiba/server,IljaN/core,endsguy/server,owncloud/core,bluelml/core,whitekiba/server,IljaN/core,pixelipo/server,cernbox/core,lrytz/core,pmattern/server,Ardinis/server,nextcloud/server,Ardinis/server,xx621998xx/server,endsguy/server,endsguy/server,sharidas/core,phil-davis/core,xx621998xx/server,endsguy/server,sharidas/core,michaelletzgus/nextcloud-server,andreas-p/nextcloud-server,cernbox/core,pmattern/server,owncloud/core,owncloud/core,IljaN/core,xx621998xx/server,whitekiba/server,lrytz/core,jbicha/server,endsguy/server,pollopolea/core,cernbox/core,jbicha/server,cernbox/core,pmattern/server,nextcloud/server,pollopolea/core,andreas-p/nextcloud-server,pixelipo/server,andreas-p/nextcloud-server,pixelipo/server,xx621998xx/server,sharidas/core