fbcon_scrub: return success to get notcurses-demo working in framebuffer

src/lib/linux.c

@@ -73,7 +73,7 @@ error:
 int fbcon_scrub(const struct ncpile* p, sprixel* s){
   (void)p;
   (void)s;
-  return -1;
+  return 0;
 }
 
 int fbcon_draw(const struct ncpile *p, sprixel* s, FILE* out, int y, int x){