libtraceevent: Change users plugin directory

libtraceevent: Change users plugin directory

To be compliant with XDG user directory layout, the user's plugin
directory is changed from ~/.traceevent/plugins to
~/.local/lib/traceevent/plugins/

Suggested-by: Patrick McLean <chutzpah@gentoo.org>
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Patrick McLean <chutzpah@gentoo.org>
Cc: linux-trace-devel@vger.kernel.org
Link: https://lore.kernel.org/linux-trace-devel/20190313144206.41e75cf8@patrickm/
Link: http://lore.kernel.org/linux-trace-devel/20190801074959.22023-4-tz.stoyanov@gmail.com
Link: http://lore.kernel.org/lkml/20190805204355.344622683@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>