perf tools: Add "reject" option for parse-events.l

Arnaldo reported broken builds in some distros using a newer flex
release, 2.6.4, found in Alpine Linux 3.6 and Edge, with flex not
spotting the REJECT macro:

  CC       /tmp/build/perf/util/parse-events-flex.o
  util/parse-events.l: In function 'parse_events_lex':
  /tmp/build/perf/util/parse-events-flex.c:4734:16: error: \
  'reject_used_but_not_detected' undeclared (first use in this function)

It's happening because we put the REJECT under another USER_REJECT macro
in following commit:

  9445464bb8 perf tools: Unwind properly location after REJECT

Fortunately flex provides option for force it to use REJECT, adding it
to parse-events.l.

Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Andi Kleen <andi@firstfloor.org>
Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: 9445464bb8 ("perf tools: Unwind properly location after REJECT")
Link: http://lkml.kernel.org/n/tip-7kdont984mw12ijk7rji6b8p@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.l

@@ -5,6 +5,7 @@
 %option stack
 %option bison-locations
 %option yylineno
+%option reject
 
 %{
 #include <errno.h>