os/KeyValueStore: fix warning

./os/KeyValueStore.h: In member function ‘std::string KeyValueStore::strip_object_key(uint64_t)’:
warning: ./os/KeyValueStore.h:173:31: format ‘%ld’ expects argument of type ‘long int’, but argument 4 has type ‘uint64_t {aka long long unsigned int}’ [-Wformat=]

Signed-off-by: Sage Weil <sage@inktank.com>