add set_brightness in framebuffer.h (#1659)

old-commit-hash: 8ad1135e80f93b19bb2f0a6b4c7531a9b5eb7d8a

selfdrive/ui/linux.cc

@@ -64,6 +64,8 @@ void framebuffer_swap(FramebufferState *s) {
   glfwPollEvents();
 }
 
+bool set_brightness(int brightness) { return true; }
+
 void touch_init(TouchState *s) {
   printf("touch_init\n");
 }