rgw: include string header

rgw: include string header

Otherwise we get complaints from the compiler on FreeBS/Clang:
In file included from /home/jenkins/workspace/ceph-master/src/rgw/rgw_sync_module_es.cc:4:
/home/jenkins/workspace/ceph-master/src/rgw/rgw_b64.h:28:22: error: implicit instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
  inline std::string to_base64(std::string_view sview)
                     ^
/usr/include/c++/v1/iosfwd:210:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_string;
                               ^
In file included from /home/jenkins/workspace/ceph-master/src/rgw/rgw_sync_module_es.cc:4:
/home/jenkins/workspace/ceph-master/src/rgw/rgw_b64.h:51:17: error: implicit instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
    std::string outstr(b64_iter(sview.data()),
                ^
/usr/include/c++/v1/iosfwd:210:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_string;

.............. more errors
                               ^
/home/jenkins/workspace/ceph-master/src/rgw/rgw_sync_module_es.cc:193:59: error: no matching function for call to 'to_base64'
      default_headers.emplace("AUTHORIZATION", "Basic " + rgw::to_base64(auth_string));
                                                          ^~~~~~~~~~~~~~
/home/jenkins/workspace/ceph-master/src/rgw/rgw_b64.h:28:22: note: candidate template ignored: substitution failure [with wrap_width = 2147483647]
  inline std::string to_base64(std::string_view sview)

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>