perf build: Fix check-headers.sh opts assignment

Currently the "opts" variable is not zero-ed and we keep on adding to
it, ending up with:

  $ check-headers.sh 2>&1
  + opts=' "-B"'
  + opts=' "-B" "-B"'
  + opts=' "-B" "-B" "-B"'
  + opts=' "-B" "-B" "-B" "-B"'
  + opts=' "-B" "-B" "-B" "-B" "-B"'
  + opts=' "-B" "-B" "-B" "-B" "-B" "-B"'

Fix this by initializing it in the check() function, right before
starting the loop.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180321140515.2252-1-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>