qt5charts: fix license information

qt5charts-2.1.3 (build with Qt 5.6) doesn't contain any license files so
remove LICENSE.GPL3 from QT5CHARTS_LICENSE_FILES when building Qt 5.6.

Fixes:
 http://autobuild.buildroot.net/results/c95d34129902a0511b8d64c9d50f1ffa293b33fd
 http://autobuild.buildroot.net/results/b9c14ce7c2fdaa412eb6416ae12a0376aaf05770
 http://autobuild.buildroot.net/results/1bde69c515d9140ef0f9a5571475c2850c7b8dee

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 9086de8e15d98abda64850c52b66756646e0c2f4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qt5/qt5charts/qt5charts.mk

@@ -11,7 +11,9 @@ QT5CHARTS_DEPENDENCIES = qt5base
 QT5CHARTS_INSTALL_STAGING = YES
 
 QT5CHARTS_LICENSE = GPL-3.0
+ifeq ($(BR2_PACKAGE_QT5_VERSION_LATEST),y)
 QT5CHARTS_LICENSE_FILES = LICENSE.GPL3
+endif
 
 ifeq ($(BR2_PACKAGE_QT5DECLARATIVE),y)
 QT5CHARTS_DEPENDENCIES += qt5declarative