tracing/boot: Fix config dependency for synthedic event

Since commit 726721a5 ("tracing: Move synthetic events to
a separate file") decoupled synthetic event from histogram,
boot-time tracing also has to check CONFIG_SYNTH_EVENT instead
of CONFIG_HIST_TRIGGERS.

Link: http://lkml.kernel.org/r/159262475441.185015.5300725180746017555.stgit@devnote2

Fixes: 726721a5

 ("tracing: Move synthetic events to a separate file")
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>