Make it simpler to build netgraph modules outside of the kernel source tree.  This change follows similar ones in the device tree.
Make it simpler to build netgraph modules outside of the kernel source
tree.  This change follows similar ones in the device tree.

MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase