perf evsel: Add no-libtraceevent stubs for evsel__field() and evsel__common_field()

perf evsel: Add no-libtraceevent stubs for evsel__field() and evsel__common_field()

When building without libtraceevent (NO_LIBTRACEEVENT=1), evsel__field()
and evsel__common_field() are declared but never defined, causing link
errors in any code path that references them.

Add inline stubs that return NULL when HAVE_LIBTRACEEVENT is not defined,
matching the pattern used by other evsel accessor functions.

Cc: Aaron Tomlin <atomlin@atomlin.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>