make clean fails on Ubuntu 16.04

[nembery]

Hi great tool!

make clean fails on Ubuntu 16.04. Appears to be variable expansion with the {,.json,.schema.json} line.

root@ubuntu-io6-28:~/libvirt-hook-qemu# make clean
rm /etc/libvirt/hooks/hooks{.json,.schema.json}
rm: cannot remove '/etc/libvirt/hooks/hooks{.json,.schema.json}': No such file or directory
Makefile:12: recipe for target 'clean' failed
make: *** [clean] Error 1

Manually expanding those results in:
rm ${LIBVIRT_HOOKS_DIR}/hooks
rm ${LIBVIRT_HOOKS_DIR}/hooks.json
rm ${LIBVIRT_HOOKS_DIR}/hooks.schema.json
and this works as expected.