tracing: Fix build breakage without CONFIG_PERF_EVENTS

Today's -next fails to build for me:

  CC      kernel/trace/trace_export.o
In file included from kernel/trace/trace_export.c:197: kernel/trace/trace_entries.h:58: error: 'perf_ftrace_event_register' undeclared here (not in a function)
make[2]: *** [kernel/trace/trace_export.o] Error 1
make[1]: *** [kernel/trace] Error 2
make: *** [kernel] Error 2

because as of ced390 (ftrace, perf: Add support to use function
tracepoint in perf) perf_trace_event_register() is declared in trace.h
only if CONFIG_PERF_EVENTS is enabled but I don't have that set.

Ensure that we always have a definition of perf_trace_event_register()
by making the definition unconditional.

Link: http://lkml.kernel.org/r/1330426967-17067-1-git-send-email-broonie@opensource.wolfsonmicro.com

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -836,13 +836,11 @@ extern const char *__stop___trace_bprintk_fmt[];
 		     filter)
 #include "trace_entries.h"
 
-#ifdef CONFIG_PERF_EVENTS
 #ifdef CONFIG_FUNCTION_TRACER
 int perf_ftrace_event_register(struct ftrace_event_call *call,
 			       enum trace_reg type, void *data);
 #else
 #define perf_ftrace_event_register NULL
 #endif /* CONFIG_FUNCTION_TRACER */
-#endif /* CONFIG_PERF_EVENTS */
 
 #endif /* _LINUX_KERNEL_TRACE_H */