package/iputils: fix rdisc server option

Commit 95b0078cc089b48be8973b9507599cc836397869 wrongly added
BUILD_RDISC_SERVER instead of ENABLE_RDISC_SERVER:

WARNING: Unknown options: "BUILD_RDISC_SERVER"

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/iputils/iputils.mk

@@ -23,7 +23,7 @@ IPUTILS_CONF_OPTS += \
 	-DBUILD_CLOCKDIFF=$(if $(BR2_PACKAGE_IPUTILS_CLOCKDIFF),true,false) \
 	-DBUILD_RARPD=$(if $(BR2_PACKAGE_IPUTILS_RARPD),true,false) \
 	-DBUILD_RDISC=$(if $(BR2_PACKAGE_IPUTILS_RDISC),true,false) \
-	-DBUILD_RDISC_SERVER=$(if $(BR2_PACKAGE_IPUTILS_RDISC_SERVER),true,false) \
+	-DENABLE_RDISC_SERVER=$(if $(BR2_PACKAGE_IPUTILS_RDISC_SERVER),true,false) \
 	-DBUILD_TRACEPATH=$(if $(BR2_PACKAGE_IPUTILS_TRACEPATH),true,false) \
 	-DBUILD_TRACEROUTE6=$(if $(BR2_PACKAGE_IPUTILS_TRACEROUTE6),true,false) \
 	-DBUILD_NINFOD=$(if $(BR2_PACKAGE_IPUTILS_NINFOD),true,false) \