PGMap: fix -Wsign-compare warning

Fix -Wsign-compare compiler warning:

mon/PGMap.cc: In member function 'void PGMap::apply_incremental
 (CephContext*, const PGMap::Incremental&)':
mon/PGMap.cc:247:30: warning: comparison between signed and
 unsigned integer expressions [-Wsign-compare]

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>