Appveyor: cast between incompatible function types in compat_time

The latest Appveyor compiler complains: {{{ ../src/lib/time/compat_time.c:522:25: error: cast between incompatible function types from 'FARPROC' to 'ULONGLONG (attribute((stdcall)) *)(void)' [-Werror=cast-function-type] 522 | GetTickCount64_fn = (GetTickCount64_fn_t) | ^ }}}

This issue is like #27465 (moved):

GetProcAddress() returns FARPROC, which is (long long int(*)()) on 64-bit Windows: https://msdn.microsoft.com/en-us/library/windows/desktop/ms683212(v=vs.85).aspx

But GetTickCount64() is (long long unsigned int(*)()), on both 32-bit and 64-bit Windows: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount64

So gcc 8 issues a spurious "incompatible function pointer" warning about the cast to GetTickCount64_fn_t.

Silence this warning by casting to a void function pointer, before the cast to GetTickCount64_fn_t.

Fixes bug NNNNN; bugfix on 0.2.9.1-alpha.