libobs/util: Add get_win_ver_int() func (windows)

Returns the current version number of windows.  The lowest byte is the
minor version number, then the next lowest byte is the major version
number.  E.g. 0x601 for Windows 7, 0x602 for Windows 8, 0x603 for
Windows 8.1, and 0xA00 for Windows 10.