perf expr: Fix "has_event" function for metric style events

perf expr: Fix "has_event" function for metric style events

Events in metrics cannot use '/' as a separator, it would be
recognized as a divide, so they use '@'. The '@' is recognized in the
metricgroups code and changed to '/', do the same in the has_event
function so that the parsing is only tried without the @s.

Fixes: 4a4a9bf9075f ("perf expr: Add has_event function")
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Kaige Ye <ye@kaige.org>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240209204947.3873294-3-irogers@google.com