perf_events: Fix modular build

Fix:

  ERROR: "perf_swevent_put_recursion_context" [fs/ext4/ext4.ko] undefined!
  ERROR: "perf_swevent_get_recursion_context" [fs/ext4/ext4.ko] undefined!

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Jason Baron <jbaron@redhat.com>
LKML-Reference: <1258864015-10579-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -3903,11 +3903,13 @@ int perf_swevent_get_recursion_context(int **recursion)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(perf_swevent_get_recursion_context);
 
 void perf_swevent_put_recursion_context(int *recursion)
 {
 	(*recursion)--;
 }
+EXPORT_SYMBOL_GPL(perf_swevent_put_recursion_context);
 
 static void __do_perf_sw_event(enum perf_type_id type, u32 event_id,
 			       u64 nr, int nmi,