games/openbor: unbreak on DragonFly after r428004

In file included from source/webmlib/halloc/halloc.c:20:0:
source/webmlib/halloc/align.h:42:25: error: conflicting types for 'max_align_t'
typedef union max_align max_align_t;

^

In file included from source/webmlib/halloc/halloc.h:18:0,

from source/webmlib/halloc/halloc.c:19:

/usr/include/stddef.h:74:3: note: previous declaration of 'max_align_t' was here
} max_align_t;

^