package/freeswitch: Fix added license

Noted by Thomas:
http://lists.busybox.net/pipermail/buildroot/2016-April/158864.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: add comma to separate the licenses.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/freeswitch/freeswitch.mk

@@ -161,7 +161,7 @@ FREESWITCH_PRE_CONFIGURE_HOOKS += FREESWITCH_ENABLE_MODULES
 # mod_isac supports a limited set of archs
 # src/mod/codecs/mod_isac/typedefs.h
 ifeq ($(BR2_i386)$(BR2_mips)$(BR2_mipsel)$(BR2_mips64)$(BR2_mips64el)$(BR2_x86_64),y)
-FREESWITCH_LICENSE += BSD-3c (mod_isac)
+FREESWITCH_LICENSE := $(FREESWITCH_LICENSE), BSD-3c (mod_isac)
 FREESWITCH_LICENSE_FILES += src/mod/codecs/mod_isac/LICENSE
 FREESWITCH_ENABLED_MODULES += codecs/mod_isac
 endif