Introduce typedef for SSL session cache callbacks

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 1a04f6cb..67fcebf 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4167,9 +4167,9 @@
 
 #if defined(MBEDTLS_SSL_SRV_C)
 void mbedtls_ssl_conf_session_cache( mbedtls_ssl_config *conf,
-        void *p_cache,
-        int (*f_get_cache)(void *, mbedtls_ssl_session *),
-        int (*f_set_cache)(void *, const mbedtls_ssl_session *) )
+                                     void *p_cache,
+                                     mbedtls_ssl_cache_get_t *f_get_cache,
+                                     mbedtls_ssl_cache_set_t *f_set_cache )
 {
     conf->p_cache = p_cache;
     conf->f_get_cache = f_get_cache;