tests: net: ppp: Compile test with PPP statistics

Make sure the compilation is ok if PPP statistics is enabled.

This is related to commit 3e704256
("net: ppp: stats: Fix net_mgmt request handler format") which
was needed because earlier commit 5a9a39ca
("net: mgmt: Convert the mgmt API to use 64-bit masks")
missed an compile problem because there was no compile test that
enabled CONFIG_NET_STATISTICS_PPP option.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>