bug#21953: Eliminate warnings in the emacs-25 release branch

[Eli Zaretskii]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Wed, 18 Nov 2015 13:39:34 -0500
> Cc: 21953@debbugs.gnu.org
> 
> > server.el:1705:1:Warning: the function 
> > ‘cygwin-convert-file-name-from-windows’
> 
> I think I just fixed this one, but someone building on a system other 
> than Cygwin will have to confirm.

Confirmed.

However, if you test for cygwin-convert-file-name-from-windows being
bound, do you still need the test for Cygwin in addition to that?