qextserialport: change license file

It's now LICENSE.md, fixes:
http://autobuild.buildroot.net/results/3e0/3e0f954df9196e89043f4aaad154dfdece08480a/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/qextserialport/qextserialport.mk

@@ -7,7 +7,7 @@
 QEXTSERIALPORT_VERSION = ada321a9ee463f628e7b781b8ed00ff219152158
 QEXTSERIALPORT_SITE = $(call github,qextserialport,qextserialport,$(QEXTSERIALPORT_VERSION))
 QEXTSERIALPORT_LICENSE = MIT
-QEXTSERIALPORT_LICENSE_FILES = LICENSE
+QEXTSERIALPORT_LICENSE_FILES = LICENSE.md
 QEXTSERIALPORT_INSTALL_STAGING = YES
 
 ifeq ($(BR2_PACKAGE_QT),y)