Fix visual selection after left click on tabline

After left clicking on a tab in the tabline, the "in_tab_line" variable wasn't
set to 'false' and every following mouse action assumed still being on the
tabline which messed up visual selection etc.

src/nvim/normal.c

@@ -2063,6 +2063,7 @@ do_mouse (
   } else if (is_drag && in_tab_line) {
     c1 = TabPageIdxs[mouse_col];
     tabpage_move(c1 <= 0 ? 9999 : c1 - 1);
+    in_tab_line = false;
     return false;
   }