iproute2: Fixes an issue with cross-compilation

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
diff --git a/Makefile b/Makefile
index ce8dd06..74e9d62 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,5 @@
 DESTDIR=/usr/
+ROOTDIR=$(DESTDIR)
 LIBDIR=/usr/lib/
 SBINDIR=/sbin
 CONFDIR=/etc/iproute2
@@ -7,7 +8,7 @@
 ARPDDIR=/var/lib/arpd
 
 # Path to db_185.h include
-DBM_INCLUDE:=/usr/include
+DBM_INCLUDE:=$(ROOTDIR)/usr/include
 
 DEFINES= -DRESOLVE_HOSTNAMES -DLIBDIR=\"$(LIBDIR)\"