core/serial: Fix warning when on platform other than linux

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/serial.c b/src/serial.c
index e61edb7..bc64b8a 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -204,8 +204,8 @@
 int
 osmo_serial_clear_custom_baudrate(int fd)
 {
-	int rc;
 #ifdef __linux__
+	int rc;
 	struct serial_struct ser_info;
 
 	rc = ioctl(fd, TIOCGSERIAL, &ser_info);