Include endian.h for missing definitions of htobe16

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/systemd/util.c
+++ b/systemd/util.c
@@ -28,6 +28,7 @@
 #include <stdbool.h>
 #include <assert.h>
 #include <errno.h>
+#include <endian.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <net/if.h>
