w32: Avoid conflict with Mingw-w64 version 4.0.4-1

* src/w32-util.c (mkstemp): Rename to my_mkstemp.  Change caller.

For some reason the linker seems to use the mkstemp now provided by
mingw instead of our static symbol. Strange.

• Reported-by: Andrej Kacian <andrej@kacian.sk>
• Signed-off-by: Werner Koch <wk@gnupg.org>