Build: Silence shift-negative-value warning

Pixman's headers include a representation of -1 in fixed-point, which is
-1 << 16. This trips a GCC warning about shifting negative values. As we
can't do much about it, just silence the warning.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
1 file changed