knock: fix _SITE so the correct git version is used

And not just the latest commit from master.

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

package/knock/knock.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 KNOCK_VERSION = 7666f2e86e18d482eaad5fe1fea46d87d80b0555
-KNOCK_SITE = $(call github,jvinet,knock,master)
+KNOCK_SITE = $(call github,jvinet,$(KNOCK_VERSION))
 KNOCK_LICENSE = GPLv2+
 KNOCK_LICENSE_FILES = COPYING
 KNOCK_AUTORECONF = YES