video/openvr: define OPENVR_API_NODLL

.. so that S_API doesn't expand to __declspec(dllimport)

src/video/openvr/SDL_openvrvideo.h

@@ -13,6 +13,7 @@
 #pragma GCC diagnostic ignored "-Wunused-variable"
 #endif
 
+#define OPENVR_API_NODLL
 #define USE_SDL
 #include "openvr_capi.h"