fix type warning

foo.diff

diff --git src/_pylibmcmodule.c src/_pylibmcmodule.c
index 98ec2f0..6cae4ef 100644
--- src/_pylibmcmodule.c
+++ src/_pylibmcmodule.c
@@ -2212,7 +2212,7 @@ static int _key_normalized_obj(PyObject **key) {
 static int _key_normalized_str(char **str, Py_ssize_t *size) {
     /* libmemcached pads max_key_size with one byte for null termination */
     if (*size >= MEMCACHED_MAX_KEY) {
-        PyErr_Format(PyExc_ValueError, "key length %d too long, max is %d",
+        PyErr_Format(PyExc_ValueError, "key length %zd too long, max is %d",
                                        *size, MEMCACHED_MAX_KEY - 1);
         return 0;
     }