Fix compilation when MISSING_LEAP_SECONDS is set to true

[techtim]

When define MISSING_LEAP_SECONDS throws compile time error: 'load_just_leaps' was not declared in this scope
when find_read_and_leap_seconds tries to call load_just_leaps which surrounded with #if !MISSING_LEAP_SECONDS
This commit surrounds call to load_just_leaps

[HowardHinnant]

What are the symptoms that are being fixed? Compile-time error? Run-time error?

I would've guessed that if this code couldn't find leap second information it would just return the empty vector.

[techtim]

compile time error: 'load_just_leaps' was not declared in this scope

[HowardHinnant]

Thanks!