windows: Do not cast FARPROC to LPVOID and then to some function pointer

windows: Do not cast FARPROC to LPVOID and then to some function pointer

FARPROC is function pointer type intptr_t(__stdcall*)() and LPVOID is data
pointer type void*. Casting from function pointer to data pointer and back
is undefined in C, and moreover in all cases it is not needed. In all cases
it is just needed to cast FARPROC function pointer type to some specific
function pointer type, and casting via intermediate LPVOID was there just
to mute compiler warnings about casting between two incompatible function
pointer types. To mute that compiler warning, do casting via intermediate
generic function pointer type void(*)(void) which is preferred according to
gcc documentation and does not throw any compiler warnings neither by gcc,
nor by msvc compilers.