Fix infinite loop in is_linked_with_cygwin_dll

Fix infinite loop in is_linked_with_cygwin_dll

There were some Windows timeouts after the last merge.  Debugging
showed that these were caused by an infinite loop in
is_linked_with_cygwin_dll when reading C:\Windows\SysWOW64\win32u.dll.

This patch fixes the problem by ensuring that the loop always makes
progress.

gdb/ChangeLog
2020-04-23  Tom Tromey  <tromey@adacore.com>

	* windows-tdep.c (is_linked_with_cygwin_dll): Always update "iter"
	in loop.