perf env: Conditionally compile BPF support code on having HAVE_LIBBPF_SUPPORT

If libbpf isn't selected, no need for a bunch of related code, that were
not even being used, as code using these perf_env methods was also
enclosed in HAVE_LIBBPF_SUPPORT.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>