route: Fix compile warning with glibc

route: Fix compile warning with glibc

This fixes the following compile problem:
````
/route.c: In function 'rtnl_flush':
/route.c:45:15: error: ignoring return value of 'write' declared with attribute 'warn_unused_result' [-Werror=unused-result]
   45 |         (void)write(fd, "-1", 2);
      |               ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.
````

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>