tools/ceph_dedup: remove 'using namespace std'

tools/ceph_dedup: remove 'using namespace std'

Remove 'using namespace std' from common.h to maintain consistent coding
practices. Although common.h is only used by ceph_dedup implementation,
keeping namespace declarations out of header files prevents potential
name conflicts and follows best practices for C++ code organization.

This change improves code clarity and reduces the risk of symbol collisions
when standard library elements are used alongside custom
implementations.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>