avoid duplicate increment of slc

Clang reported "warning: variable 'slc' is incremented both in the loop
header and in the loop body".

src/ustreamer/stream.c

@@ -127,7 +127,6 @@ void us_stream_loop(us_stream_s *stream) {
  unsigned slc = 0;
  for (; slc < 10 && !atomic_load(&_RUN(stop)) && !us_stream_has_clients(stream); ++slc) {
  usleep(100000);
- ++slc;
  }
  h264_force_key = (slc == 10);
  }