Makefile: remove pkg-stats data on clean

Makefile: remove pkg-stats data on clean

Like commit 1f187371d002 for cpe-updates data, also remove pkg-stats
data on clean.

Unlike the rest, those are not nicely located in a directory of their
own, and have no variable name associated with them, so we just need
to repeat their names in the clean rule.

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: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>