lib/buildstats: fix parsing of trees with reduced_proc_pressure directories

The /proc/pressure support in buildstats is creating directories in the
buildstats tree called reduced_proc_pressure, which confuses the parsing
logic as that cannot be parsed as a name-epoc-version-revision tuple.

Explicitly skip this directory to solve the problem.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 24f0331f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>