Fixed coverity issue in acl_context.cpp: Type: AUTO_CAUSES_COPY (AUTO_CAUSES_COPY)

Changed to `auto &_context` since the previous code `auto _context` would only make a copy of the contexts in acl_platform.contexts_set and the subsequent `acl_update_context(_context)` would not have done anything to the contexts in acl_platform.contexts_set.

Author: haoxian2

src/acl_context.cpp

@@ -1202,7 +1202,7 @@ void acl_idle_update(cl_context context) {
   // firstly update the current context
   acl_update_context(context);
   // update the other contexts from the platform
-  for (auto _context : acl_platform.contexts_set) {
+  for (const cl_context _context : acl_platform.contexts_set) {
     if (context != _context)
       acl_update_context(_context);
   }