wimax (plugins): fix no previous prototype for function 'proto_reg_handoff_wimax' [-Wmissing-prototypes]

Change-Id: Icc79994ace4e32def3f03cf62b49f3c7dd3011df
Reviewed-on: https://code.wireshark.org/review/25384
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Michael Mann <mmann78@netscape.net>

plugins/epan/wimax/packet-wmx.c

@@ -37,6 +37,7 @@
 #include "wimax_utils.h"
 
 void proto_register_wimax(void);
+void proto_reg_handoff_wimax(void);
 
 /* Global variables */
 gint	proto_wimax = -1;