c++: add missing va_end

Found via cppcheck.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>

src/syncevo/SyncContext.cpp

@@ -4887,6 +4887,7 @@ private:
             }
         }
         list.syncDone(status, &report);
+        va_end(ap);
 
         Logger::instance().setLevel(level);
         return list.getLogdir();