common/bit_vector.hpp: do not move a temporary object

common/bit_vector.hpp: do not move a temporary object

this change also silences a waring from clang:

src/common/bit_vector.hpp:278:21: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
      m_data.append(std::move(buffer::ptr(buffer_size - m_data.length())));
                    ^
../src/common/bit_vector.hpp:278:21: note: remove std::move call here
      m_data.append(std::move(buffer::ptr(buffer_size - m_data.length())));
                    ^~~~~~~~~~                                          ~

Signed-off-by: Kefu Chai <kchai@redhat.com>