uefisign: fix SizeOfHeaders sanity check.

This check was too aggressive: it is fine if SizeOfHeaders is exactly
equal to the size of the DOS stub + PE header + section table. Despite
being wrong this code typically worked for most EFI binaries because
SizeOfHeaders is rounded up to a multiple of FileAlignment, which is
often large (e.g., 512 bytes for the FreeBSD loader) so most binaries
made it through.

Reviewed by: imp@
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/445