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

Changed to `const cl_context _context` since coverity complains about the previous code auto _context would make an auto copy of the pointer contexts in acl_platform.contexts_set. Because `_context` is a pointer, a copy is equally as cheap as a reference, and to pass coverity which only mentions `auto` copy.

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);
   }