logger: Fix compile error
logger: Fix compile error

gcc version 5.1.1 20150618 (Red Hat 5.1.1-4) (GCC)

[1/6] CXX build/release/log.o
FAILED: g++ -MMD -MT build/release/log.o -MF build/release/log.o.d
-std=gnu++1y -g  -Wall -Werror -fvisibility=hidden -pthread -I.
-U_FORTIFY_SOURCE  -I/usr/include/jsoncpp/ -Wno-maybe-uninitialized
-DHAVE_XEN -DHAVE_HWLOC -DHAVE_NUMA -O2 -I build/release/gen -c -o
build/release/log.o log.cc
log.cc: In member function ‘void
logging::logger::really_do_log(logging::log_level, const char*,
logging::logger::stringer**, size_t)’:
log.cc:86:19: error: no match for ‘operator<<’ (operand types are
‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::ostringstream
{aka std::basic_ostringstream<char>}’)
         std::cout << out;
                   ^
log.cc:86:19: note: candidate: operator<<(int, int) <built-in>
log.cc:86:19: note:   no known conversion for argument 2 from
‘std::ostringstream {aka std::basic_ostringstream<char>}’ to ‘int’
In file included from /usr/include/c++/5.1.1/iostream:39:0,
                 from core/sstring.hh:31,
                 from log.hh:8,
                 from log.cc:5:

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