Fix engine on iOS

[B3rn475]

Related #4105

[aam]

The patch have fixed compilation error that I got at the tip:

/Source/vsync_waiter_ios.mm -o obj/flutter/shell/platform/darwin/ios/libFlutter/vsync_waiter_ios.o
../../flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm:65:49: error: invalid operands to binary expression ('fxl::TimePoint' and 'int')
    sprintf(deadline, "%lld", frame_target_time / 1000);  // microseconds
                              ~~~~~~~~~~~~~~~~~ ^ ~~~~

[aam]

So LGTM