libqmlbind-sys: Disable 'improper_ctypes' warning
libqmlbind-sys: Disable 'improper_ctypes' warning

Suprresses all warnings:
    warning: found zero-size struct in foreign module, consider adding a
    member to this struct, #[warn(improper_ctypes)] on by default

See also #2.

nbigaouette/qmlrsng,nbigaouette/qmlrsng