tools/rebuild_mondb: kill compiling warning

tools/rebuild_mondb: kill compiling warning

As follow:

[ 72%] Building CXX object src/tools/CMakeFiles/ceph-objectstore-tool.dir/RadosDump.cc.o
/home/jenkins-build/build/workspace/ceph-pull-requests/src/tools/rebuild_mondb.cc: In function ‘int update_mon_db(ObjectStore&, OSDSuperblock&, const string&, const string&)’:
/home/jenkins-build/build/workspace/ceph-pull-requests/src/tools/rebuild_mondb.cc:289:22: warning: ‘crc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         if (have_crc && osdmap.get_crc() != crc) {
                      ^
/home/jenkins-build/build/workspace/ceph-pull-requests/src/tools/rebuild_mondb.cc:238:14: note: ‘crc’ was declared here
     uint32_t crc;

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>