ssl_test_lib: move declaration of query_config

Move from ssl_*2.c to ssl_test_lib.h:
* The declaration of query_config(). Also document it.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

programs/ssl/ssl_client2.c

@@ -521,8 +521,6 @@ struct options
     const char *mki;            /* The dtls mki value to use                */
 } opt;
 
-int query_config( const char *config );
-
 #if defined(MBEDTLS_SSL_EXPORT_KEYS)
 typedef struct eap_tls_keys
 {