package/ibrdtnd: fix build with gcc 11

Fixes:
 - No autobuilder failures yet

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

package/ibrdtnd/ibrdtnd.mk

@@ -18,7 +18,9 @@ IBRDTND_CONF_OPTS = \
 	--without-vmime
 
 # don't build documentation
-IBRDTND_CONF_ENV = PDFLATEX='no'
+IBRDTND_CONF_ENV = \
+	PDFLATEX='no' \
+	CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
 
 ifeq ($(BR2_PACKAGE_LIBDAEMON),y)
 IBRDTND_CONF_OPTS += --enable-libdaemon