Fix missing error message when device global name is not unique

The stringstream err_ss is only used within the function, and the
contents must be assigned to err_str which is returned to the caller.

This amends #100

Signed-off-by: Peter Colberg <peter.colberg@intel.com>

Author: pcolberg

src/acl_auto_configure.cpp

@@ -559,6 +559,7 @@ bool acl_load_device_def_from_str(const std::string &config_str,
         // unique.
         err_ss << "Device global name should be unique. " << device_global_name
                << " is repeated.\n";
+        err_str = err_ss.str();
         result = false;
       }