net: fix ifenslave build flags

-I (include path) should be specified for host builds.
This one was overlooked somehow.  Fixes
https://bugzilla.kernel.org/show_bug.cgi?id=25902

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Alexey Salmin <alexey.salmin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/Makefile

@@ -4,6 +4,8 @@ obj- := dummy.o
 # List of programs to build
 hostprogs-y := ifenslave
 
+HOSTCFLAGS_ifenslave.o += -I$(objtree)/usr/include
+
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)