xdriver_xf86-video-sis: use += instead of = for CONF_OPTS assignment

Using += is always a good idea in conditional assignments, as it
prevents potential future mistakes if unconditional options are added;

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/x11r7/xdriver_xf86-video-sis/xdriver_xf86-video-sis.mk

@@ -13,7 +13,7 @@ XDRIVER_XF86_VIDEO_SIS_AUTORECONF = YES
 XDRIVER_XF86_VIDEO_SIS_DEPENDENCIES = xserver_xorg-server libdrm xproto_fontsproto xproto_randrproto xproto_renderproto xproto_videoproto xproto_xextproto xproto_xf86dgaproto xproto_xf86driproto xproto_xineramaproto xproto_xproto
 
 ifeq ($(BR2_PACKAGE_MESA3D_DRI_DRIVER),)
-XDRIVER_XF86_VIDEO_SIS_CONF_OPTS = --disable-dri
+XDRIVER_XF86_VIDEO_SIS_CONF_OPTS += --disable-dri
 endif
 
 $(eval $(autotools-package))