osd/PG.h: remove unused variable prior_set_built

Commit 540f42ce284d9 removed usage of prior_set_built, remove
also the related protected variable from the header.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
osd/PG.cc: initialize PG::flushed in constructor

Initialize PG::flushed in constructor with false as
described in doc/dev/osd_internals/pg.rst .

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #218 from dalgaaf/wip-da-sca-cppcheck

Two small fixes

Reviewed-by: Sage Weil <sage@inktank.com>