libcacard: Fix compilation with gcc-4.7

VCARD_ATR_PREFIX is used as part of an array initializer so it should
not have () around it, so far this happened to work, but gcc-4.7 does
not like it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
