extcap-base.c: fix compilation with gcc 8

extcap-base.c: fix compilation with gcc 8

extcap-base.c:293:43: error: cast between incompatible function types from ‘void (*)(void *)’ to ‘void (*)(void *, void *)’ [-Werror=cast-function-type]
extcap-base.c:302:45: error: cast between incompatible function types from ‘void (*)(void *)’ to ‘void (*)(void *, void *)’ [-Werror=cast-function-type]
extcap-base.c:321:42: error: cast between incompatible function types from ‘void (*)(void *)’ to ‘void (*)(void *, void *)’ [-Werror=cast-function-type]

Change-Id: Ifd9151d04412c5e29636dc14c57a327cc12d33f3
Reviewed-on: https://code.wireshark.org/review/27265
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Reviewed-by: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>