gitignore: ignore generated hz.bc file

Commit 70730bca causes kernel/hz.bc to be autogenerated at build time.
Let git ignore the file so it won't show up in git status.
Signed-off-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/.gitignore

@@ -4,3 +4,4 @@
 config_data.h
 config_data.gz
 timeconst.h
+hz.bc