blktrace: the ftrace interface needs CONFIG_TRACING

Impact: build fix

Also mention in the help text that blktrace now can be used using
the ftrace interface.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

block/Kconfig

@@ -50,6 +50,8 @@ config BLK_DEV_IO_TRACE
 	select RELAY
 	select DEBUG_FS
 	select TRACEPOINTS
+	select TRACING
+	select STACKTRACE
 	help
 	  Say Y here if you want to be able to trace the block layer actions
 	  on a given queue. Tracing allows you to see any traffic happening
@@ -58,6 +60,12 @@ config BLK_DEV_IO_TRACE
 
 	  git://git.kernel.dk/blktrace.git
 
+	  Tracing also is possible using the ftrace interface, e.g.:
+
+	    echo 1 > /sys/block/sda/sda1/trace/enable
+	    echo blk > /sys/kernel/debug/tracing/current_tracer
+	    cat /sys/kernel/debug/tracing/trace_pipe
+
 	  If unsure, say N.
 
 config BLK_DEV_BSG