hyperv: erase "generated files" sentinel during make maintainer-clean

hyperv: erase "generated files" sentinel during make maintainer-clean

Re-generating of generated source files in the hyperv directory
depends on src/.hyperv_wmi_generator.stamp not existing, or having a
timestamp older than src/hyperv/hyperv_wmi_generator.py. "make
maintainer-clean" erases the generated files, but not this sentinel
file, so the erased files aren't regenerated during the next
make. Once we add it to the list of MAINTAINERCLEANFILES, it gets
deleted at the same time as the generated files, so make is able to
understand they need regeneration.

Signed-off-by: Laine Stump <laine@laine.org>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>