libmicrohttpd: fix license file setting

The variable is called <pkg>_LICENSE_FILES, not just _FILE.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libmicrohttpd/libmicrohttpd.mk

@@ -6,7 +6,7 @@
 
 LIBMICROHTTPD_VERSION = 0.9.27
 LIBMICROHTTPD_SITE = $(BR2_GNU_MIRROR)/libmicrohttpd
-LIBMICROHTTPD_LICENSE_FILE = COPYING
+LIBMICROHTTPD_LICENSE_FILES = COPYING
 LIBMICROHTTPD_INSTALL_STAGING = YES
 
 ifeq ($(BR2_PACKAGE_LIBMICROHTTPD_SSL),y)