win32sck.c: use "fd_set" instead of "FD_SET"

The former is the canonical name of the type. The latter doesn't work
on MinGW.org.

win32/win32sck.c

@@ -492,7 +492,7 @@ win32_select(int nfds, Perl_fd_set* rd, Perl_fd_set* wr, Perl_fd_set* ex, const
 {
     int r;
     int i, fd, save_errno = errno;
-    FD_SET nrd, nwr, nex;
+    fd_set nrd, nwr, nex;
     bool just_sleep = TRUE;
 
     StartSockets();