core: Fix gcc-7 -Wimplicit-fallthrough= warnings

core: Fix gcc-7 -Wimplicit-fallthrough= warnings

the comment /* Fall through. */ fixes the warning, but gcc-7 seems to be more
picky about the position in the code

pulsecore/sink-input.c: In function ‘pa_sink_input_update_proplist’:
pulsecore/sink-input.c:1531:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
             for (state = NULL; (key = pa_proplist_iterate(i->proplist, &state));) {
             ^~~
pulsecore/sink-input.c:1539:9: note: here
         case PA_UPDATE_REPLACE: {
         ^~~~

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>