perf bpf-event: Use libbpf version rather than feature check

perf bpf-event: Use libbpf version rather than feature check

The feature check guarded the -DHAVE_LIBBPF_STRINGS_SUPPORT is
unnecessary as it is sufficient and easier to use the
LIBBPF_CURRENT_VERSION_GEQ macro.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Blake Jones <blakejones@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>