[efi] Fix recorded overall size of headers in NT optional header

[efi] Fix recorded overall size of headers in NT optional header

Commit 1e4c378 ("[efi] Shrink size of data directory in PE header")
reduced the number of entries used in the data directory and reduced
the recorded size of the NT "optional" header, but did not also adjust
the recorded overall size of the PE headers, resulting in unused space
between the PE headers and the first section.

Fix by reducing the initial recorded size of the PE headers by the
size of the omitted data directory entries.

Signed-off-by: Michael Brown <mcb30@ipxe.org>