README.source
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