Fix compiler warnings when buliding on AppVeyor

Fix compiler warnings when buliding on AppVeyor

../mtucalc.c: In function 'calculate_mtu':
../mtucalc.c:75:33: warning: passing argument 4 of 'getsockopt' from incompatible pointer type [-Wincompatible-pointer-types]
   75 |                                 &mss, &mss_size)) {
      |                                 ^~~~
      |                                 |
      |                                 int *
In file included from ../openconnect-internal.h:31,
                 from ../mtucalc.c:20:
C:/msys64/mingw64/include/winsock2.h:1010:82: note: expected 'char *' but argument is of type 'int *'
 1010 |   WINSOCK_API_LINKAGE int WSAAPI getsockopt(SOCKET s,int level,int optname,char *optval,int *optlen);
      |                                                                            ~~~~~~^~~~~~
  CC       libopenconnect_la-lzo.lo

../cstp.c: In function 'calculate_dtls_mtu':
../cstp.c:134:33: warning: passing argument 4 of 'getsockopt' from incompatible pointer type [-Wincompatible-pointer-types]
  134 |                                 &mss, &mss_size)) {
      |                                 ^~~~
      |                                 |
      |                                 int *
In file included from ../openconnect-internal.h:31,
                 from ../cstp.c:21:
C:/msys64/mingw64/include/winsock2.h:1010:82: note: expected 'char *' but argument is of type 'int *'
 1010 |   WINSOCK_API_LINKAGE int WSAAPI getsockopt(SOCKET s,int level,int optname,char *optval,int *optlen);
      |                                                                            ~~~~~~^~~~~~

Signed-off-by: Dimitri Papadopoulos Orfanos <3350651-DimitriPapadopoulos@users.noreply.gitlab.com>