gpgscm: Fix compact vector encoding.

* tests/gpgscm/scheme-private.h (struct cell): Use uintptr_t for
'_flags'.  This way, '_flags' has the size of a machine word.

The compact vector representation introduced in 49e2ae65 requires that
we can tell apart pointers and type flags. This did not work on
64-bit big-endian architectures.

Fixes a crash on 64-bit big-endian architectures.

• Hat-tip-to: gniibe
• Fixes-commit: 49e2ae65e892f93be7f87cfaae3392b50a99e4b1
• Signed-off-by: Justus Winter <justus@g10code.com>