Fix fall-through comment in windows-nat.c

Fix fall-through comment in windows-nat.c

Now that the mingw builder in the buildbot is working again, it
pointed out a build failure due to a missing fall-through comment in
windows-nat.c.  This patch fixes the problem.

Tested by first triggering the failure with a local mingw build, then
by rebuilding successfully with the patch.

I'm checking this in as obvious.

gdb/ChangeLog
2018-05-29  Tom Tromey  <tom@tromey.com>

	* windows-nat.c (handle_exception): Update fall-through comment.