tools lib traceevent: Add tep_load_plugins_hook() API

tools lib traceevent: Add tep_load_plugins_hook() API

Add the API function tep_load_plugins_hook() to the traceevent API to
allow tools a common method to load in the plugins that are part of the
lib traceevent library.

Link: http://lore.kernel.org/linux-trace-devel/20190802110101.14759-4-tz.stoyanov@gmail.com
Link: http://lore.kernel.org/linux-trace-devel/20200625100516.365338-4-tz.stoyanov@gmail.com
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200702185703.946652691@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>