Add a (void*) cast to GetProcAddress

[tomrittervg]

Visual C++ has an extension that auto-magically converts a FARPROC to a void*. On MinGW we get an error like:

In function ‘void* SharedLib_GetFunction(SharedLibHandle, const char*)’: openvr/src/sharedlibtools_public.cpp:25:23: error: invalid conversion from ‘FARPROC {aka int (attribute((stdcall)) )()}’ to ‘void’ [-fpermissive] return GetProcAddress( (HMODULE)lib, pchFunctionName );

See also http://stackoverflow.com/questions/13958081/mingw-compile-error-invalid-conversion-from-farproc-to-void-but-msvc-com

[natbro]

thanks - good fix - we'll fold this into the next drop & close it then after you confirm.