try to unbreak `perf`

I'm getting an error message `Couldn't synthesize bpf events.`, and the
resulting data file from `perf record` apparently contains garbage. At
least it's rejected by `perf report`:

 WARNING: The perf.data file's data size field is 0 which is unexpected.

and `hotspot` allocates a garbage amount of memory and dies.

Change-Id: Ib19fd04471ee8400fa5341c28874f17d764bbbeb
diff --git a/board/czechlight/clearfog/linux.fragment b/board/czechlight/clearfog/linux.fragment
index 9844d2b..2ad46e8 100644
--- a/board/czechlight/clearfog/linux.fragment
+++ b/board/czechlight/clearfog/linux.fragment
@@ -50,5 +50,6 @@
 CONFIG_MV643XX_ETH=n
 CONFIG_KEYBOARD_ATKBD=n
 CONFIG_INPUT_MOUSE=n
+CONFIG_BPF_SYSCALL=y
 
 CONFIG_LOCALVERSION="-6-ge8ddee461538"