Fix “initialization from incompatible pointer type” for default_options.

Fix “initialization from incompatible pointer type” for default_options.

InputDriverRec's last field is a char**, not a const char**, fix this
warning accordingly:
|   CC     acecad.lo
| acecad.c:132:1: warning: initialization from incompatible pointer type [enabled by default]
| acecad.c:132:1: warning: (near initialization for 'ACECAD.default_options') [enabled by default]

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Cyril Brulebois <kibi@debian.org>