avrdude: 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/avrdude/avrdude.mk

@@ -7,7 +7,7 @@
 AVRDUDE_VERSION = ad04c429a90f4c34f000ea4ae11db2705915a31f
 AVRDUDE_SITE = $(call github,kcuzner,avrdude,$(AVRDUDE_VERSION))
 AVRDUDE_LICENSE = GPLv2+
-AVRDUDR_LICENSE_FILES = avrdude/COPYING
+AVRDUDE_LICENSE_FILES = avrdude/COPYING
 AVRDUDE_SUBDIR = avrdude
 # Sources coming from git, without generated configure and Makefile.in
 # files.