[makeotfexe] DFLT script being the first languagesystem is not always enforced

[khaledhosny]

The spec (§4.b.i) states that:

If the statement languagesystem DFLT dflt; is present, it must be the first of the languagesystem statements.

But makeotfexe enforces this only if there is more than one languagesystem before DFLT. The following passes without error after removing the first languagesystem statement:

languagesystem latn dflt;
languagesystem latn dflt;
languagesystem DFLT dflt;

feature liga {
    sub f l by f_l;
} liga;