perfcounters: add context switch counter

Impact: add new feature, new sw counter

Add a counter that counts the number of context-switches a task
is doing.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index e5d25bf..d2a1656 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -53,8 +53,8 @@
 	/*
 	 * Future software events:
 	 */
-	/* PERF_COUNT_PAGE_FAULTS	= -3,
-	   PERF_COUNT_CONTEXT_SWITCHES	= -4, */
+	PERF_COUNT_PAGE_FAULTS		= -3,
+	PERF_COUNT_CONTEXT_SWITCHES	= -4,
 };
 
 /*