Fix old thinko in pgp.h: the idea is to declare some named enum types,

Fix old thinko in pgp.h: the idea is to declare some named enum types,
not global variables of anonymous enum types.  This didn't actually hurt
much because most linkers will just merge the duplicated definitions ...
but some will complain.  Per bug #4731 from Ceriel Jacobs.

Backpatch to 8.1 --- the declarations don't exist before that.