graphics/sane-frontends: Unbreak after r517720

gtkglue.c:1479:24: error: use of undeclared identifier 'SANE_CAP_ALWAYS_SETTABLE'

if (!(opt->cap & SANE_CAP_ALWAYS_SETTABLE))
                 ^

1 error generated.

PR: 241225
Reported by: cmt
Obtained from: Fedora
Pointy hat: tobik