build: the reference trees are not ours

fcitx5, fcitx5-hangul, fcitx5-gtk, kime and libhangul are cloned into the
work tree as ref-*/ to be read against, so git has to be told to leave them
alone.  One pattern rather than five names: whatever gets cloned next is
covered too, and none of it is ever ours to track.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -18,3 +18,4 @@
 /gtk/im-strans.so
 /bench/bench
 /bench/perf.data*
+/ref-*/