Add guards for MBEDTLS_X509_CRL_PARSE_C in sample

Add checks in `ssl_server2` that `MBEDTLS_X509_CRL_PARSE_C` is defined
to fix compilation issue. Fixes #560.
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index ec23c8a..e8e5cd1 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -229,8 +229,14 @@
 #endif /* MBEDTLS_SSL_CACHE_C */
 
 #if defined(SNI_OPTION)
+#if defined(MBEDTLS_X509_CRL_PARSE_C)
+#define SNI_CRL              ",crl"
+#else
+#define SNI_CRL              ""
+#endif
+
 #define USAGE_SNI                                                           \
-    "    sni=%%s              name1,cert1,key1,ca1,crl1,auth1[,...]\n"  \
+    "    sni=%%s              name1,cert1,key1,ca1"SNI_CRL",auth1[,...]\n"  \
     "                        default: disabled\n"
 #else
 #define USAGE_SNI ""
@@ -565,10 +571,10 @@
 
         mbedtls_x509_crt_free( cur->ca );
         mbedtls_free( cur->ca );
-
+#if defined(MBEDTLS_X509_CRL_PARSE_C)
         mbedtls_x509_crl_free( cur->crl );
         mbedtls_free( cur->crl );
-
+#endif
         next = cur->next;
         mbedtls_free( cur );
         cur = next;
@@ -587,7 +593,10 @@
     sni_entry *cur = NULL, *new = NULL;
     char *p = sni_string;
     char *end = p;
-    char *crt_file, *key_file, *ca_file, *crl_file, *auth_str;
+    char *crt_file, *key_file, *ca_file, *auth_str;
+#if defined(MBEDTLS_X509_CRL_PARSE_C)
+    char *crl_file;
+#endif
 
     while( *end != '\0' )
         ++end;
@@ -605,7 +614,9 @@
         GET_ITEM( crt_file );
         GET_ITEM( key_file );
         GET_ITEM( ca_file );
+#if defined(MBEDTLS_X509_CRL_PARSE_C)
         GET_ITEM( crl_file );
+#endif
         GET_ITEM( auth_str );
 
         if( ( new->cert = mbedtls_calloc( 1, sizeof( mbedtls_x509_crt ) ) ) == NULL ||
@@ -630,6 +641,7 @@
                 goto error;
         }
 
+#if defined(MBEDTLS_X509_CRL_PARSE_C)
         if( strcmp( crl_file, "-" ) != 0 )
         {
             if( ( new->crl = mbedtls_calloc( 1, sizeof( mbedtls_x509_crl ) ) ) == NULL )
@@ -640,6 +652,7 @@
             if( mbedtls_x509_crl_parse_file( new->crl, crl_file ) != 0 )
                 goto error;
         }
+#endif
 
         if( strcmp( auth_str, "-" ) != 0 )
         {