perf: parse-events: Fix compilation error while defining DEBUG_PARSER

Compiling perf tool with 'DEBUG_PARSER=1' leads to errors:

$> make -C tools/perf PARSER_DEBUG=1 NO_LIBTRACEEVENT=1
...
  CC      util/expr-flex.o
  CC      util/expr.o
util/parse-events.c:33:12: error: redundant redeclaration of ‘parse_events_debug’ [-Werror=redundant-decls]
   33 | extern int parse_events_debug;
      |            ^~~~~~~~~~~~~~~~~~
In file included from util/parse-events.c:18:
util/parse-events-bison.h:43:12: note: previous declaration of ‘parse_events_debug’ with type ‘int’
   43 | extern int parse_events_debug;
      |            ^~~~~~~~~~~~~~~~~~
util/expr.c:27:12: error: redundant redeclaration of ‘expr_debug’ [-Werror=redundant-decls]
   27 | extern int expr_debug;
      |            ^~~~~~~~~~
In file included from util/expr.c:11:
util/expr-bison.h:43:12: note: previous declaration of ‘expr_debug’ with type ‘int’
   43 | extern int expr_debug;
      |            ^~~~~~~~~~
cc-1: all warnings being treated as errors

Remove extern declaration from the parse-envents.c file as there is a
conflict with the ones generated using bison and yacc tools from the file
parse-events.[ly].

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240605140453.614862-1-clement.legoffic@foss.st.com