Patch from Colin Watson intended to give a clean Unix compile with GCC 4.

Patch from Colin Watson intended to give a clean Unix compile with GCC 4.
(Since we choose to compile with -Werror, this is particularly important.)

I haven't yet checked that the resulting source actually compiles cleanly with
GCC 4, hence not marking `gcc4-warnings' as fixed just yet.

git-svn-id: http://svn.tartarus.org/sgt/putty@7041 cda61777-01e9-0310-a592-d414129be87e