Fix alignment issue in uefisign

The pe_certificate structure has to be aligned to 8 bytes. [1]
Since this is now checked in edk2, any binaries signed with
older version of this tool will fail verification.

References:
[1] https://docs.microsoft.com/en-us/windows/desktop/Debug/pe-format#the-attribute-certificate-table-image-only

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: trasz
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D18554