[rpcrt4] Fix RpcMgmtSetServerStackSize prototype (2nd attempt)

[Vitaliy Margolen]

Thomas Weidenmueller wrote:
> The parameter of RpcMgmtSetServerStackSize needs to be unsigned long,
> not unsigned int.
> 
> - Thomas
> 
It's been discussed numerous times, you should avoid use of 'long' in
Wine. On win64 sizeof(long) == 4. On Linux64 sizeof(long) == 8.

Vitaliy.