[util] Use HB_FALLTHROUGH

[util] Use HB_FALLTHROUGH

Sure, gcc knows to warn about this as well:

../../util/options.cc:175:17: warning: this statement may fall through [-Wimplicit-fallthrough=]
     case 1: m.r = m.t;
             ~~~~^~~~~
../../util/options.cc:176:5: note: here
     case 2: m.b = m.t;
     ^~~~

But HOLY SMOKES, look at clang -Weverything bot message:

options.cc:176:5: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
    case 2: m.b = m.t;
    ^
options.cc:176:5: note: insert 'HB_FALLTHROUGH;' to silence this warning
    case 2: m.b = m.t;
    ^
    HB_FALLTHROUGH;

Right, it's telling me to insert "HB_FALLTHROUGH;" there!!!!!!!!!