vlc: fix typo when setting VLC_CONF_ENV

Fixes:
  http://autobuild.buildroot.org/results/917/917c26656557d50f869639162ec7e9d0f3399824/build-end.log

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/vlc/vlc.mk

@@ -211,7 +211,7 @@ endif
 ifeq ($(BR2_PACKAGE_LIVE555),y)
 VLC_CONF_OPT += --enable-live555
 VLC_DEPENDENCIES += live555
-VLC_CONF_ENV = \
+VLC_CONF_ENV += \
 	LIVE555_CFLAGS="\
 		-I$(STAGING_DIR)/usr/include/live \
 		-I$(STAGING_DIR)/usr/include/live/BasicUsageEnvironment \