Cast GetVolumeInformationByHandleW to (void *)

Cast GetVolumeInformationByHandleW to (void *)

The 64-bit Windows build was complaining of pointer type mismatches but
I'm fairly sure it was a false positive:

../ssl.c: In function 'openconnect_passphrase_from_fsid':
../ssl.c:598:9: warning: cast between incompatible function types from 'FARPROC' {aka 'long long int (*)()'} to 'BOOL (*)(void *, WCHAR *, DWORD,  DWORD *, DWORD *, DWORD *, WCHAR *, DWORD)' {aka 'int (*)(void *, short unsigned int *, long unsigned int,  long unsigned int *, long unsigned int *, long unsigned int *, short unsigned int *, long unsigned int)'} [-Wcast-function-type]
  598 |  func = (GVIBH)GetProcAddress(kernlib, "GetVolumeInformationByHandleW");
      |         ^

Signed-off-by: David Woodhouse <dwmw2@infradead.org>