package/apcupsd: don't override dependencies

Don't override APCUPSD_DEPENDENCIES in a conditional

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/apcupsd/apcupsd.mk

@@ -34,7 +34,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_APCUPSD_MODBUS_USB),y)
 APCUPSD_CONF_OPTS += --enable-modbus-usb
-APCUPSD_DEPENDENCIES = libusb libusb-compat
+APCUPSD_DEPENDENCIES += libusb libusb-compat
 else
 APCUPSD_CONF_OPTS += --disable-modbus-usb
 endif
@@ -65,7 +65,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_APCUPSD_USB),y)
 APCUPSD_CONF_OPTS += --enable-usb
-APCUPSD_DEPENDENCIES = libusb libusb-compat
+APCUPSD_DEPENDENCIES += libusb libusb-compat
 else
 APCUPSD_CONF_OPTS += --disable-usb
 endif