Add: Mock sprite cache intialization.

This fills up the sprite cache with SPR_OPENTTD_BASE + OPENTTD_SPRITE_COUNT zero-size sprites, to
allow GetSpriteSize() calls to continue from unit-tests.

src/spritecache_internal.h

@@ -36,6 +36,7 @@ static inline bool IsMapgenSpriteID(SpriteID sprite)
 	return IsInsideMM(sprite, SPR_MAPGEN_BEGIN, SPR_MAPGEN_END);
 }
 
+void *AllocSprite(size_t mem_req);
 SpriteCache *AllocateSpriteCache(uint index);
 
 #endif /* SPRITECACHE_INTERNAL_H */