gpgscm: Make tags mandatory.

* tests/gpgscm/opdefines.h: Make tags mandatory.
* tests/gpgscm/scheme.c: Likewise.
* tests/gpgscm/scheme.h: Likewise.

Tags provide a constant-time lookup mechanism for almost every object.
This is useful for the interpreter itself, and the code for tags is
tiny.

• Signed-off-by: Justus Winter <justus@g10code.com>