ext: nffs: update for enabling statistic

Origin: Apache Mynewt NFFS
URL: https://github.com/apache/mynewt-nffs/tree/master


Commit: 199575aa8b309dfe809cc59b9c6854ccdad9bdf7
Maintained-by: External

This patch introduce version whitch enables NFFS statistic.
Statistic functionalities were (recently) introduced within
mgmtm subsystem.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>