simple_counter::add(): Add a type cast to the os_atomic_increment_ulint()
call, because GCC would check the type compatibility even when the code
branch is not being instantiated (atomic=false). On Solaris,
os_atomic_increment_ulint() actually needs a compatible parameter type,
and an error would be emitted due to an incompatible 64-bit type,
for srv_stats.n_lock_wait_time.add(diff_time).