batman-adv: Drop unused soft-interface.h include in fragmentation.c

The commit 992b03b8 ("batman-adv: Don't always reallocate the
fragmentation skb head") removed the last user of functions from
soft-interface.h.
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/fragmentation.c

@@ -27,7 +27,6 @@
 #include "originator.h"
 #include "routing.h"
 #include "send.h"
-#include "soft-interface.h"
 
 /**
  * batadv_frag_clear_chain() - delete entries in the fragment buffer chain