configure: Fix whitespaces for the --gen-se-header help text

Replace some of the tabs with spaces to display the help text for the
--gen-se-header option like this:

    --gen-se-header=GEN_SE_HEADER
                           Provide an executable to generate a PV header
                           requires --host-key-document. (s390x-snippets only)

instead of:

    --gen-se-header=GEN_SE_HEADER
                           Provide an executable to generate a PV header
   requires --host-key-document. (s390x-snippets only)

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>