include: pe.h: add signature-related definitions

include: pe.h: add signature-related definitions

The index (IMAGE_DIRECTORY_ENTRY_SECURITY) in a table points to
a region containing authentication information (image's signature)
in PE format.

WIN_CERTIFICATE structure defines an embedded signature format.

Those definitions will be used in my UEFI secure boot patch.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>