[classify] Workaround to detect exe from sample cache initated sink-inputs.

Sample cache initiated sink-inputs don't have a client, but sample's proplist
contains PA_PROP_APPLICATION_PROCESS_BINARY anyway. Try to get this value
from proplist. This allows using 'exe' matching in stream definitions in xpolicy.conf
for sample cache initiated streams as well.
Merge pull request #11 from jusa/sample-cache

Workaround to detect exe from sample cache initated sink-inputs.