perf env: Move perf_env out of header.h and session.c into separate object

perf env: Move perf_env out of header.h and session.c into separate object

Since it can be used separately from 'perf_session' and 'perf_header',
move it to separate include file and object, next csets will try to move
a perf_env__init() routine.

Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-ff2rw99tsn670y1b6gxbwdsi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>