common/Formatter: include used header

common/Formatter: include used header

otherwise it fails to build with gcc-toolset-10, like:

../src/common/Formatter.cc: In member function ‘virtual void ceph::XMLFormatter::close_section()’:
../src/common/Formatter.cc:449:8: error: ‘transform’ is not a member of ‘std’
  449 |   std::transform(section.begin(), section.end(), section.begin(),
      |        ^~~~~~~~~

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