fix compilation on Windows

Fixes compilation of server-win.c, so that at least why3 can be compiled on Windows (which enables opam to install dependent packages such as Frama-C).

Note that there are still other issues with Why3 on Windows, at least when using it via MinGW + Cygwin. For instance, I'm unable to get it to run Alt-Ergo (bad escape sequence '\O' in string: C:\OCaml64\home\user\.opam\4.14.0+mingw64c\bin\alt-ergo.exe --timelimit %.t %f), and commands such as why3 webserver return anomaly: Invalid_argument("Sys.signal: unavailable signal").

Therefore, I was unable to test that my patch does provide the expected behavior for the VC server; I copied the pattern from elsewhere, but without being able to test it, I'm not entirely sure it works as expected. But at least compilation works.