Use SDL C runtime functions in OpenVR driver

Fixes link errors when building with -DSDL_LIBC=OFF

src/video/openvr/SDL_openvrvideo.c

@@ -911,7 +911,7 @@ static SDL_GLContext OPENVR_GL_CreateContext(SDL_VideoDevice *_this, SDL_Window
 
     OPENVR_SetupFrame(_this, window);
 
-    SDL_GLContext result = malloc(sizeof(struct SDL_GLContextState));
+    SDL_GLContext result = SDL_malloc(sizeof(struct SDL_GLContextState));
     if (!result) {
         return NULL;
     }