Repair bad manual cherrypick of vsync repair

src/sdl/i_video.c

@@ -101,6 +101,8 @@ rendermode_t rendermode = render_none;
 
 boolean highcolor = false;
 
+static void Impl_SetVsync(void);
+
 // synchronize page flipping with screen refresh
 consvar_t cv_vidwait = {"vid_wait", "Off", CV_SAVE|CV_CALL|CV_NOINIT, CV_OnOff, Impl_SetVsync, 0, NULL, NULL, 0, 0, NULL};
 static consvar_t cv_stretch = {"stretch", "Off", CV_SAVE|CV_NOSHOWHELP, CV_OnOff, NULL, 0, NULL, NULL, 0, 0, NULL};
@@ -2137,3 +2139,5 @@ static void Impl_SetVsync(void)
 	if (renderer)
 		SDL_RenderSetVSync(renderer, cv_vidwait.value);
 #endif
+}
+#endif