python-keyring: unbreak after download rework

commit 514291f39ef06 (packages: use new $($PKG)_DL_DIR) variable) strangely
enough removed the PYTHON_KEYRING_SITE line, breaking the build.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python-keyring/python-keyring.mk

@@ -6,6 +6,7 @@
 
 PYTHON_KEYRING_VERSION = 10.5.0
 PYTHON_KEYRING_SOURCE = keyring-$(PYTHON_KEYRING_VERSION).tar.gz
+PYTHON_KEYRING_SITE = https://pypi.python.org/packages/42/2e/51bd1739fe335095a2174db3f2f230346762e7e572471059540146a521f6
 PYTHON_KEYRING_SETUP_TYPE = setuptools
 PYTHON_KEYRING_LICENSE = MIT
 PYTHON_KEYRING_LICENSE_FILES = LICENSE