Fix build failure on Linux.

(cherry picked from commit ef14050f)

src/systemd.c

@@ -37,6 +37,8 @@
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>
+#include <fcntl.h>
+
 #include <unistd.h>
 
 #include <netinet/in.h>