Real sources are in the  package illumos-source-X.Y,
where X and Y are numbers. Exact values of X and Y
are set in Build-Depends field of debian/control as
well as in the version string in debian/changelog,
in a form of X.Y-1.

To add a patch you need first to unpack files from
illumos-source-X.Y using ./debian/rules unpack
and, desirably, ./debian/rules patch