package/ibrdtn-tools: 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/ibrdtn-tools/ibrdtn-tools.mk

@@ -9,6 +9,7 @@ IBRDTN_TOOLS_SITE = https://www.ibr.cs.tu-bs.de/projects/ibr-dtn/releases
 IBRDTN_TOOLS_LICENSE = Apache-2.0
 IBRDTN_TOOLS_LICENSE_FILES = COPYING
 IBRDTN_TOOLS_DEPENDENCIES = ibrcommon ibrdtn host-pkgconf
+IBRDTN_TOOLS_CONF_ENV = CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
 
 ifeq ($(BR2_STATIC_LIBS),y)
 IBRDTN_TOOLS_CONF_ENV += LDFLAGS="$(TARGET_LDFLAGS) -pthread"