Pkcs7VerifyDxe: Don't allow Pkcs7Verify to install protocols twice

Pkcs7VerifyDxe: Don't allow Pkcs7Verify to install protocols twice

This patch makes Pkcs7VerifyDxe check that it has not already been
installed before installing its protocols.  This prevents the case
where loading it as an external driver (either manually, through
Driver#### variables, etc.) will refuse to add a second provider of
the API.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Long Qin <qin.long@intel.com>