CLEANUP: exclude haring with .gitignore

haring is a new utility to decode file-backed rings. It is compiled in
dev/ directory and so the binary should be specified in .gitignore to
not clutter git status output.
diff --git a/.gitignore b/.gitignore
index a277688..1844c4f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -47,6 +47,7 @@
 dev/flags/flags
 dev/poll/poll
 dev/tcploop/tcploop
+dev/haring/haring
 dev/hpack/decode
 dev/hpack/gen-rht
 dev/qpack/decode