Add '=for comment ifdef' to pod pages

Make find-doc-nits understand that
        =for comment ifdef ssl3 ...
in a POD page means that the "-ssl3" flag might be ifdef'd out in the
local environment, and not to complain about it.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9974)
diff --git a/doc/man1/openssl-genrsa.pod b/doc/man1/openssl-genrsa.pod
index b963dd7..81ede1b 100644
--- a/doc/man1/openssl-genrsa.pod
+++ b/doc/man1/openssl-genrsa.pod
@@ -31,6 +31,8 @@
 [B<-verbose>]
 [B<numbits>]
 
+=for comment ifdef engine
+
 =head1 DESCRIPTION
 
 The B<genrsa> command generates an RSA private key.