on2-8170-libs: fix typo in variable name

As pointed out during the check-package discussion, there is a typo in a
variable name:

http://lists.busybox.net/pipermail/buildroot/2016-December/180765.html

Reported-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/on2-8170-libs/on2-8170-libs.mk

@@ -7,7 +7,7 @@
 ON2_8170_LIBS_VERSION = 1.0
 ON2_8170_LIBS_SITE = ftp://ftp.linux4sam.org/pub/demo/linux4sam_1.9/codec
 
-ON2_8170_LICENSE = PROPRIETARY
+ON2_8170_LIBS_LICENSE = PROPRIETARY
 # No license file is included in the archive
 
 ON2_8170_LIBS_INSTALL_STAGING = YES