in6: move include

Move the include for sysctl.h out of the middle of the file to the
includes at the beginning.  This is will make it easier to add new
sysctls.

No functional changes.

MFC after:	3 weeks
Sponsored by:	Netflix

sys/netinet6/in6.c

@@ -84,6 +84,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/rmlock.h>
+#include <sys/sysctl.h>
 #include <sys/syslog.h>
 
 #include <net/if.h>
@@ -2024,8 +2025,6 @@ in6_if2idlen(struct ifnet *ifp)
 	}
 }
 
-#include <sys/sysctl.h>
-
 struct in6_llentry {
 	struct llentry		base;
 };