fix for [perl #61976] Errno ($!) not evaluated to a error message string

fix for [perl #61976] Errno ($!) not evaluated to a error message string
(5.10.0 in taint mode)

Change 27176 / 2a509ed3c095f7d712013e653f68821f6bb2d6db fixed a taint
bug, which as a side effect, meant that $! used within a tainted expression
failed to have a string value.

This quick fix just makes sure the POK flag is set (prior to it being
shifted back to pPOK).