input/tablet: fix `wl_array_for_each` usage on tablet proximity
input/tablet: fix `wl_array_for_each` usage on tablet proximity

Looks like this instance was missed in
e035f2b9c42b39e3eff37d0fe98bfa6422877d7a.

Fixes #3110.

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots