MessageQueue: add missing enum value to the '@StackNodeType'.

'STACK_NODE_QUITTING' constant was add in ag/33468905, however it
wasn't add to the list IntDefs of '@StackNodeType'. In this CL we
add it, to fix the linter warnings.

Test: Pure refactoring.
Flag: EXEMPT Pure refactoring.
Bug: N/A
Change-Id: I392f37d1b842d654eff1b342c6c4a6207c7c8eff