aufs6: fix magic.mk include path

commit 482d7131560c51da3ca [aufs stdalone: make it standalone] in the
aufs upstream changes the way that the .mk file is included.

This doesn't work properly with all kernel build processes, so we
restore the use of srctree/src to locate the file.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>