package/libftdi: don't override license files variable

Fixes:
  package/libftdi/libftdi.mk:22: conditional override of variable LIBFTDI_LICENSE_FILES

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 080fadc115914e84113cc5f67853b65d8a5ce844)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libftdi/libftdi.mk

@@ -19,7 +19,7 @@ LIBFTDI_CONF_OPTS = --without-examples
 # configure detect it automaticaly so we need to force it
 ifeq ($(BR2_PACKAGE_LIBFTDI_CPP),y)
 LIBFTDI_LICENSE += , GPL-2.0 with exception (ftdipp)
-LIBFTDI_LICENSE_FILES = COPYING.GPL
+LIBFTDI_LICENSE_FILES += COPYING.GPL
 LIBFTDI_DEPENDENCIES += boost
 LIBFTDI_CONF_OPTS += --enable-libftdipp
 else