Fix misdef of platform_get_x11_unix_address on Windows.

Similarly to the previous commit, this function had an inconsistent
parameter list between Unix and Windows, because the Windows source
file that defines it (winnet.c) didn't include ssh.h where its
prototype lives, so the compiler never checked.

Luckily, the discrepancy was that the Windows version of the function
was declared as taking an extra parameter which it ignored, so the fix
is very easy.

(cherry picked from commit b7f011aed7)

windows/winnet.c

@@ -1817,8 +1817,7 @@ char *get_hostname(void)
     return dupstr(hostbuf);
 }
 
-SockAddr *platform_get_x11_unix_address(const char *display, int displaynum,
-                                       char **canonicalname)
+SockAddr *platform_get_x11_unix_address(const char *display, int displaynum)
 {
     SockAddr *ret = snew(SockAddr);
     memset(ret, 0, sizeof(SockAddr));