perf jevents metric: Fix type of strcmp_cpuid_str

perf jevents metric: Fix type of strcmp_cpuid_str

The parser wraps all strings as Events, so the input is an
Event. Using a string would be bad as functions like Simplify are
called on the arguments, which wouldn't be present on a string.

Fixes: 9d5da30e4ae9 ("perf jevents: Add a new expression builtin strcmp_cpuid_str()")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20230914022204.1488383-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>