performance: App not installable in testing

In testing performance app is not installable. Cockpit has removed cockpit-pcp app and cockpit-bridge now performs that duty. cockpit-bridge package provides 'cockpit-pcp'. However, when trying to install from FreedomBox, application shows as unavailable.

Further, cockpit-bridge needs 'pcp' package to be installed separately. It does not depend on the pcp package like cockpit-pcp did.

'pcp' package is currently not installable in Debian testing due to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1071103. Upgrading the package to version 7.x seems to fix the bug but 7.x has not been uploaded yet.