Missing VREvent_t and ButtonMaskFromId in openvr_capi.h

[lukexi]

I'm able to work around this by pasting these in manually after the #include.

struct VREvent_t
{
  EVREventType eventType;
  TrackedDeviceIndex_t trackedDeviceIndex;
  VREvent_Data_t data;
  float eventAgeSeconds;
};

inline uint64_t ButtonMaskFromId( EVRButtonId id ) { return 1ull << id; }