Dbus_Rest: Fix bmcweb crash

GET on URI "/bus/system/" causes bmcweb service to crash.
This occurred as asyncResp was captured by reference by lambda.

Tested:
 - GET on /bus/system/ responded with desired output without crash.

Signed-off-by: P Dheeraj Srujan Kumar <p.dheeraj.srujan.kumar@intel.com>
Change-Id: I2c777be4bc2c95332d47df380c867891b133d016