Fix build of games/xinvaders on 13 and 14

There are symbols declared in vaders.h with ext as storage class qualifier.
Those are either extern or (blank) depending on MAIN being defined.
Some of them duplicate symbols that are defined in spacers.c, though.
Those ought to be extern in all cases, rather than also defined in
main.c (because the storage class qualifier is blank then).

Reported by: antoine