Fix mingw32 build: cast setsockopt optval to const char *

[fjtrujy]

Summary

• PR Enable TCP_NODELAY #22 added a setsockopt(..., TCP_NODELAY, &one, sizeof(one)) call that breaks the mingw32 build because Winsock's setsockopt prototype takes const char *optval (not const void * like POSIX).
• Adds a (const char *) cast — harmless on POSIX, required on Windows.

This is what currently fails CI on windows-latest in the ps2dev meta build:

src/ps2link.c:61:58: error: passing argument 4 of 'setsockopt' from incompatible pointer type [-Wincompatible-pointer-types]
   61 |     setsockopt(request_socket, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));
note: expected 'const char *' but argument is of type 'int *'
make[1]: *** [Makefile.mingw32:26: obj/ps2link.o] Error 1

Test plan

• CI green on Linux/macOS (unchanged behavior — cast is a no-op on POSIX where optval is const void *)
• CI green on Windows / mingw32

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