[PATCH relayd] route: Fix compile warning with glibc

[Hauke Mehrtens]

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 at hauke-m.de>
---
 route.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/route.c b/route.c
index c552d1f..602fdc9 100644
--- a/route.c
+++ b/route.c
@@ -36,13 +36,16 @@ int route_table = 16800;
 
 static void rtnl_flush(void)
 {
+	ssize_t ret;
 	int fd;
 
 	fd = open("/proc/sys/net/ipv4/route/flush", O_WRONLY);
 	if (fd < 0)
 		return;
 
-	write(fd, "-1", 2);
+	ret = write(fd, "-1", 2);
+	if (ret != 2)
+		perror("write");
 	close(fd);
 }
 
-- 
2.39.0