*: add missing includes

Some places, particularly headers, were spewing warnings since they
don't include neccessary other headers to get struct/enum definitions.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/if_sysctl.c b/zebra/if_sysctl.c
index 5e80996..ffa6927 100644
--- a/zebra/if_sysctl.c
+++ b/zebra/if_sysctl.c
@@ -29,6 +29,7 @@
 #include "memory.h"
 #include "ioctl.h"
 #include "log.h"
+#include "interface.h"
 
 #include "zebra/rt.h"
 #include "zebra/kernel_socket.h"