Improve #include structure of pem{,2}.h

  - pem2.h is empty, so pem.h doesn't need to include it.
  - pem2.h once declared ERR_load_PEM_strings(), so it should now
    include pemerr.h

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5150)