crush: pass by const reference to kill compiling error

crush: pass by const reference to kill compiling error

i have following error when compiling with gcc-6 on armhf

ceph/src/crush/CrushCompiler.cc: In member function 'int
CrushCompiler::decompile(std::ostream&)':
ceph/src/crush/CrushCompiler.cc:462:45: error: invalid initialization of
non-const reference of type 'std::pair<const long unsigned int,
crush_choo
se_arg_map>&' from an rvalue of type 'std::pair<const long unsigned int,
crush_choose_arg_map>'
       int ret = decompile_choose_args(i, out);

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