Fix mingw CI failures

Change `%z` formatting of `size_t` to `%u` and casting to unsigned.
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index a33cfb5..9260fbf 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -489,9 +489,9 @@
 
     if( opt.debug_level > 2 )
     {
-        mbedtls_printf("exported maclen is %zu\n",maclen);
-        mbedtls_printf("exported keylen is %zu\n",keylen);
-        mbedtls_printf("exported ivlen is %zu\n",ivlen);
+        mbedtls_printf("exported maclen is %u\n", (unsigned)maclen);
+        mbedtls_printf("exported keylen is %u\n", (unsigned)keylen);
+        mbedtls_printf("exported ivlen is %u\n", (unsigned)ivlen);
     }
     return( 0 );
 }