Avoid "unused variable" warning on non-USE_SSL_ENGINE platforms.

If we are building with openssl but USE_SSL_ENGINE didn't get set,
initialize_SSL's variable "pkey" is declared but used nowhere.
Apparently this combination hasn't been exercised in the buildfarm
before now, because I've not seen this warning before, even though
the code has been like this a long time.  Move the declaration
to silence the warning (and remove its useless initialization).

Per buildfarm member sawshark.  Back-patch to all supported branches.

src/interfaces/libpq/fe-secure-openssl.c

@@ -902,7 +902,6 @@ initialize_SSL(PGconn *conn)
 	bool		have_homedir;
 	bool		have_cert;
 	bool		have_rootcert;
-	EVP_PKEY   *pkey = NULL;
 
 	/*
 	 * We'll need the home directory if any of the relevant parameters are
@@ -1252,6 +1251,7 @@ initialize_SSL(PGconn *conn)
 			/* Colon, but not in second character, treat as engine:key */
 			char	   *engine_str = strdup(conn->sslkey);
 			char	   *engine_colon;
+			EVP_PKEY   *pkey;
 
 			if (engine_str == NULL)
 			{