ftrace: add FTRACE_EVENT_ENABLED macro

Add a macro for checking if an ftrace event is enabled or not.
Gets as argument the name of the event, as assigned to the event in the
event declaration or definition.
Returns 1 if event is enabled, 0 otherwise.

Change-Id: Ib6c39b8c67ff20623e9e2eb7996a39285c230d03
Signed-off-by: Lee Susman <lsusman@codeaurora.org>