Makefile: remove cpe-updates on clean

Makefile: remove cpe-updates on clean

Commit fd7312940aef (Makefile: add new missing-cpe target) added the
rule to generate a set of files to update the NVD.

For an in-tree build, 'make clean' remove the output directory, so
those files are removed. But for an out-of-tree build, the output
directory is not removed, so those files still linger around after a
clean.

Explicitly remove them on clean, to cater for both cases.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: Matthew Weber <matthew.weber@collins.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>