fix killsnoop.py srr/bytes type error (#2007)

Fix issue #2002
Signed-off-by: Yonghong Song <yhs@fb.com>

tools/killsnoop.py

@@ -139,7 +139,7 @@ def print_event(cpu, data, size):
     if (args.failed and (event.ret >= 0)):
         return
 
-    printb(b"%-9s %-6d %-16s %-4d %-6d %d" % (strftime("%H:%M:%S"),
+    printb(b"%-9s %-6d %-16s %-4d %-6d %d" % (strftime("%H:%M:%S").encode('ascii'),
         event.pid, event.comm, event.sig, event.tpid, event.ret))
 
 # loop with callback to print_event