[PATCH] Fix blktrace compile with sysfs not defined

debugfs depends on sysfs, so make blktrace kconfig option depend
on that.

Reported by Adrian Bunk.

Signed-off-by: Jens Axboe <axboe@suse.de>
diff --git a/block/Kconfig b/block/Kconfig
index 43ca070..5536839 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -13,6 +13,7 @@
 
 config BLK_DEV_IO_TRACE
 	bool "Support for tracing block io actions"
+	depends on SYSFS
 	select RELAY
 	select DEBUG_FS
 	help