sim: ppc: enable -Wmissing-declarations & -Wmissing-prototypes

sim: ppc: enable -Wmissing-declarations & -Wmissing-prototypes

This aligns with common code which already uses this flag.  We have
to add another local prototype to fix the failure, and add another
local decl for the SIM_DESC type.  Unwinding these will require a
lot more work & conversions in the process, so going with the decl
for now unblocks the warning unification.