tracing: Undef TRACE_EVENT_FN between trace events headers inclusion

The recent commit:

	tracing/events: fix the include file dependencies

fixed a file dependency problem while including more than
one trace event header file.

This fix undefined TRACE_EVENT after an event header macro
preprocessing in order to make tracepoint.h able to correctly declare
the tracepoints necessary for the next event header file.

But now we also need to undefine TRACE_EVENT_FN at the end of an event
header file preprocessing for the same reason.

This fixes the following build error:

In file included from include/trace/events/napi.h:5,
                 from net/core/net-traces.c:28:
include/linux/tracepoint.h:285:1: warning: "TRACE_EVENT_FN" redefined
In file included from include/trace/define_trace.h:61,
                 from include/trace/events/skb.h:40,
                 from net/core/net-traces.c:27:
include/trace/ftrace.h:50:1: warning: this is the location of the previous definition
In file included from include/trace/events/napi.h:5,
                 from net/core/net-traces.c:28:
include/linux/tracepoint.h:285:1: warning: "TRACE_EVENT_FN" redefined
In file included from include/trace/define_trace.h:61,
                 from include/trace/events/skb.h:40,
                 from net/core/net-traces.c:27:
include/trace/ftrace.h:50:1: warning: this is the location of the previous definition

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <20090827161732.GA7618@nowhere>
Signed-off-by: Ingo Molnar <mingo@elte.hu>