Unbreak the macOS NS build

* src/xdisp.c (expose_window_tree): Declare and define 'f' on
NS as well.  Reported by Herbert J. Skuhra <herbert@gojira.at>.
1 job for master in 47 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #596
test-all

00:47:33