SecurityPkg/SecurityPkg.dec: Update PcdPkcs7CertBuffer PCD.

SecurityPkg/SecurityPkg.dec: Update PcdPkcs7CertBuffer PCD.

This patch updates the PcdPkcs7CertBuffer PCD to use the new
generated test certificate data for PKCS7 verification. This
was used as sample trusted certificate in the verification of
Signed Capsule Update.
(The updated value is still only for test purpose.)

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Long Qin <qin.long@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>