games/vkquake: fix build on 32-bits

./atomics.h:201:9: error: large atomic operation may incur significant performance penalty; the access size (8 bytes) exceeds the max lock-free size (4 bytes) [-Werror,-Watomic-alignment]

return atomic_fetch_add (atomic, value);
       ^

/usr/include/stdatomic.h:354:2: note: expanded from macro 'atomic_fetch_add'

atomic_fetch_add_explicit(object, operand, memory_order_seq_cst)
^

/usr/include/stdatomic.h:247:2: note: expanded from macro 'atomic_fetch_add_explicit'

__c11_atomic_fetch_add(object, operand, order)