This patch allows contrib/pgcrypto to build with OpenSSL 0.9.8

This patch allows contrib/pgcrypto to build with OpenSSL 0.9.8
(currently in beta) when cryptolib = openssl.  According to the
following checkin message from several years ago, OpenSSL application
developers should no longer rely on <openssl/evp.h> to include
everything they need:

http://cvs.openssl.org/chngview?cn=9888

This patch adds the necessary header files.  It doesn't appear to
break anything when building against OpenSSL 0.9.7.

BTW, core appears to build and work fine with OpenSSL 0.9.8.  I've
built 7.3 through HEAD against 0.9.8-beta6 without noticing any
problems.

Michael Fuhr