sink input: remove compilation warnings
sink input: remove compilation warnings

This change allows to get rid of a couple of compilation time warnings:

  sink-input-ext.c:358:21: warning: assignment discards qualifiers from pointer
  target type
  sink-input-ext.c:361:9: warning: passing argument 3 of 'pa_proplist_get' from
  incompatible pointer type

alinelena/pulseaudio-policy-enforcement,jusa/pulseaudio-policy-enforcement,alinelena/pulseaudio-policy-enforcement,jusa/pulseaudio-policy-enforcement