iproute2: add missing host-flex dependency

Fixes
http://autobuild.buildroot.org/results/0931e622a0e80fc7321f0e26c96ae4607eaadd00/build-end.log.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/iproute2/iproute2.mk

@@ -10,7 +10,7 @@ IPROUTE2_TARGET_SBINS = ctstat genl ifstat ip lnstat nstat routef routel rtacct
 IPROUTE2_LICENSE = GPLv2
 IPROUTE2_LICENSE_FILES = COPYING
 
-IPROUTE2_DEPENDENCIES += host-bison
+IPROUTE2_DEPENDENCIES += host-bison host-flex
 
 # If both iproute2 and busybox are selected, make certain we win
 # the fight over who gets to have their utils actually installed.